Why I Shoot .40 Through My Unmodified 10mm GLOCK


10mm Auto came first. The FBI chose a watered-down loading — a 180 grain bullet at 975 fps instead of more like 1,300 fps — and a large-frame S&W pistol through which to shoot it. By the time the contract went through, Tom Campbell, S&W employee, had realized that the powder capacity of the 10mm’s 25.2 mm-long case simply wasn’t necessary to achieve this same velocity, and the .40 S&W with its 21.6 mm-long case was born. For all intents and purposes, .40 S&W is really “10mm Short & Weak.” Brief history complete, let’s skip to the part where I’ve chosen to shoot it regularly through factory-stock 10mm GLOCKs. . .

First, yes, the disclaimer: Don’t do this. It isn’t okay to shoot a caliber in a firearm that wasn’t designed for that caliber. I’m going to tell you why I do it and why I actually believe it’s safer to shoot .40 in a 10mm GLOCK than either 10mm in a 10mm or .40 in a .40, but that doesn’t mean I’m suggesting you do it. So don’t.

Second, this all pertains to a GLOCK. Shooting .40 in many or most other 10mm pistols sans conversion barrel can be problematic or unsafe.


A competitive shooter I knew in California shot a GLOCK 20 in Production class. However, he shot .40 S&W from it. No need to shoot the stronger and significantly more expensive 10mm just to make Major power factor, yet he’s an extremely tall guy with gigantic hands and the G20 frame fit him better. Additionally, the extra mass of the gun made it shoot that much softer and flatter. He shot about 5,000 rounds per year, and was in his 4th year with the same G20 when I met him. Before the G20, he was shooting a GLOCK 22 until he encountered another bloke who was running .40 through his G20 and claimed to have put many thousands of rounds through it without a hitch.

Confirming complete and total reliability through his G20, I was also surprised to see that once the barrel was cleaned it was entirely indistinguishable from any other GLOCK barrel. Finish wear on the outside attested to the round count, but the inside was pristine.

Digging into this further, I found that it’s actually a fairly common practice among the GLOCK set and I started doing it myself. I got my G20SF for bear defense while Huckleberry picking and, yes, to shoot full-power 10mm through it, but when I’m practicing, plinking, and target shooting — or testing out new GLOCK sights like these ones that are on my G20 right now — I’m usually shooting the less expensive .40 S&W. No, I do not have a conversion barrel and I don’t feel any need whatsoever to buy one. I also had a Gen4 G29 — the sub-compact 10mm — for about a year and shot a couple hundred .40’s through that bad boy without a hitch.

Why It Works

Many people believe the extractor in a pistol snaps over the rim of the cartridge, but that’s not actually how it happens. As the bottom of the breech face strips the round out of the magazine and the feed ramp lifts the round upwards, the base of the cartridge slides up the breech face, sliding underneath the extractor. .40 S&W feeding from a 10mm magazine in a 10mm GLOCK follows this process just like it should. Since the .40 brass is 3.6 mm shorter, its forward progress isn’t stopped by the end of the 10mm chamber (it doesn’t headspace on the case mouth). Instead, the extractor hangs onto it and keeps it up against the breech face.

Obviously the bullet has a few more millimeters of free bore travel before it contacts rifling than a 10mm will, but this is far from unique in the firearms world. Every revolver necessitates bullet travel before rifling is encountered, especially in cases where you’re shooting a shorter, less-powerful version of the cartridge the revolver is designed for, like .38 Special out of a .357 Magnum or the various .32 cartridges from a .327 Federal Magnum, or, heck, a .45 LC out of a 3″-chambered Taurus Judge. Or a .22 Short out of a .22 LR.

.40 S&W has identical exterior dimensions to 10mm, uses the exact same bullets, has the same rim dimensions, etc. It uses a small pistol primer instead of a large one, is rated for 35,000 PSI instead of the 37,500 PSI of the 10mm, and is obviously a bit shorter in case length and overall loaded cartridge length. Other than “headspacing off the extractor” instead of the case mouth, everything else is functionally the same as 10mm as far as a GLOCK is concerned.

As a 10mm GLOCK is tuned from the factory to reliably run FBI-spec 10mm, it will run .40 S&W as, again, .40 was designed to be ballistically identical to FBI 10mm. With the exception of brands like Underwood, Buffalo Bore, DoubleTap, and a couple of the big names, much of the “10mm” ammo on the market is loaded dang close to a given brand’s .40 cal. For instance, Federal American Eagle and Federal Hydra-Shok have stated velocities of 1,000 fps for 180 grn .40 and 1,030 fps for 180 grn 10mm. In fact, even when I install my 21-lb (a few lbs stiffer than factory) Lone Wolf recoil spring, it still cycles .40 with complete reliability.


As mentioned, I believe shooting .40 S&W in a 10mm GLOCK is not only safe, but is safer than shooting 10mm in a 10mm GLOCK or .40 in a .40 GLOCK. Here’s why I don’t hesitate to do it:

1) Headspace. “Kabooms” can occur due to excessive headspace — the distance between the breech face and the end of the chamber (in this case the lip on which the case mouth stops) — caused by the cartridge protruding too far out the back (ejection port side) of the chamber. With fairly generous chamber diameters and less-than-complete case support — the “GLOCK Smile” appears on .40 S&W and 10mm cases when shot through the correct GLOCK models due to limited case support above the feed ramp — it doesn’t take very much excess headspace to cause a case rupture. Since these rimless cartridges are designed to headspace off the mouth of the case when it hits the front of the chamber, a piece of brass that’s slightly too long, a bullet that isn’t seated deeply enough, heavy chamber fouling, and/or a slide that doesn’t quite go fully into battery can and does cause this dangerous scenario. 10mm and .40 S&W GLOCKs are both relatively notorious for case blowouts, and this is a big reason why.

Conversely, when the shorter .40 S&W cartridge is chambered in the 10mm chamber, there’s effectively no physical way for it to be anything less than completely chambered. Even out of spec, too-long brass with a shallow-seated bullet is going to have room in the chamber and the slide is going to be completely in battery, as far forwards as it’s capable of going. The least headspace possible. Every time.

In fact, this can be seen immediately if one happens to have both a .40 and a 10mm GLOCK handy. .40 brass shot out of the .40 will show the GLOCK Smile (as does 10mm out of a 10mm). .40 brass shot out of the 10mm will not. There is no longer any GLOCK case bulge, or at least it’s greatly reduced. The case is more fully chambered and better supported. If you reload, you’ll be happy to know that fired .40 cases look and measure better when shot through a 10mm barrel (case length changes by no more or less than normal, and rims are normal).

2) Overbuilt. A 10mm GLOCK has a heavier and thicker slide, thicker chamber and barrel walls, stronger recoil spring, and a larger locking block than a .40 GLOCK. In the event of a .40 S&W round that is over pressure due to excessive powder, a bullet that is seated too deeply, etc, the pistol is more likely to shrug it off or at least more likely to suffer less damage.

3) Firing Pin. This is what makes this uniquely safe in a GLOCK but a bit sketchy in many other firearms. The big, rectangular slab of a GLOCK firing pin (striker) comes to a hard stop without projecting very far from the breech face. Should the extractor miss the case rim for some reason or slip off of it — neither of which it ever freakin’ does, by the way — the .40 S&W round would plunk down into the chamber, held forwards off of the breech face by the extractor far enough that the firing pin is physically incapable of contacting the primer.

This isn’t the case in some 1911s and other guns. While this still actually isn’t likely to cause a serious safety problem, it isn’t ideal for the case to slam backwards into the breech face and it can lead to pierced primers and worn out or broken extractors, and maybe failures to eject. The unlikely safety issue would be separation of the case rim/base from the rest of the casing. Again, it won’t happen in a GLOCK because the extractor is either holding the cartridge against the breech face and all is well, or it isn’t and the firing pin cannot reach the primer in the first place.

Primer strikes on .40 S&W cases shot through 10mm GLOCKs look indistinguishable from those shot through .40 S&W GLOCKs. This isn’t usually the case in other pistols with normally-shaped firing pins, which tend to exhibit excessively-deep primer strikes and sometimes pierced primers.

4) Extractor. The GLOCK extractor is pretty wide and beefy. A concern for extractor lifespan is probably the #1 “don’t do this because…” thing I hear on this .40/10 topic after safety concerns are dismissed. I think the theoretical stress on the extractor from having to stop the round from plunking into the chamber is exaggerated. That forwards pull is just nothing compared to how hard it sometimes has to tug on cases to pull them out of the chamber. I don’t deny that the extractor is doing work on the chambering stroke when it usually doesn’t, but I see it as much lighter work than its normal duty and, as they say, the proof is in the pudding. The proof is in the countless G20s with multiple thousands of rounds of .40 through them with no extractor breakages or issues of any sort, whatsoever. In theory, practice is the same as theory, but in practice it rarely is. It’s also like a $16 part, and it’s not like they don’t sometimes break in by-the-book usage. I don’t think this increases the chances.

Additionally, it’s sometimes the case that the correct cartridge in the correct gun doesn’t headspace on the case mouth anyway. With brass that’s only a little bit on the short side — often even within SAAMI-approved tolerances — it doesn’t contact the end of the chamber. Short is okay, long is bad (see #1 above).

EDIT: a concern was brought up in the comments about a gap between breech face and cartridge base. First, some gap there is really a non-issue on a pistol cartridge and all revolvers have some gap there (between case base and recoil shield) so the cylinder can rotate, but I should have mentioned that the GLOCK extractor pins the round against the breech face under spring tension. The extractor isn’t just hovering there over the rim, it’s actually clamped down on it. There is no gap, and the round is held solidly in place:



Note extractor is moved outwards by the case rim.

5) Chamber / throat erosion. Folks are convinced that the shorter, .40 cal brass means the gasses and bullet will impact the chamber lip and wear it out. Again, I understand it intuitively but it doesn’t pan out in practice. The gasses from the pistol cartridge just aren’t hot enough and high-pressure enough to begin to wear away the steel for many, many tens of thousands of rounds. Again, some of the time the 10mm brass isn’t going to seal against the chamber lip anyway. As the full diameter of either the 10mm or .40 projectile is behind the chamber lip, they both have the ability to touch it. The copper and lead isn’t hard enough to erode that cold hammer forged steel except for, again, after many tens of thousands of rounds exactly as it takes that sort of time for the rifling to get worn down in a standard pistol barrel.

I’ve now seen multiple factory GLOCK barrels with multiple tens of thousands of rounds of .40 S&W through them and, when clean, no GLOCK Armorer or gunsmith on the planet could tell by looking at the chamber lip or the bore that .40 was fired through it instead of 10mm.

On The Range

It’s completely reliable. I have never suffered a stoppage of any sort, with all possible combinations of factory barrel, factory recoil spring, Lone Wolf 6.6″ 10mm barrel, and Lone Wolf 21-lb recoil spring. Nor was there a hitch in the couple hundred rounds fired through the unmodified Gen4 G29 that I had. The guy that piqued my interest in this had never suffered a single .40-in-a-10-related stoppage — he encountered a few truly dud primers — and during the competition season in which we were in contact he was working on rounds ~15,000 to ~20,000.

It is not quite as accurate as 10mm in a 10mm barrel or .40 in a .40 barrel. That little bit of free bore has a very minor, detrimental effect on accuracy. Obviously it isn’t enough to negatively affect USPSA scores. The gun will still shoot groups that anyone would call typical for a GLOCK. However, I did take the time to do various testing around this and group sizes are just slightly larger than when shooting either caliber through its correct barrel.

The Legitimate Safety Concern

As the .40 brass is shorter, shooting it will cause a bit more carbon fouling than usual in the front of the chamber. It is advisable that one cleans one’s chamber before switching back to 10mm. As we’ve already discussed, excessive headspace can lead to a kaboom — especially with a GLOCK-brand GLOCK barrel — and one cause of excessive headspace can be fouling that prevents a round from fully chambering. It could take a thousand rounds of .40 through a 10mm GLOCK before fouling is such that it’s even almost a concern for switching back to 10mm, but as there are a lot of variables that can affect the rate and type of fouling there’s no way I’d throw out a suggested round count. Default should be cleaning, mandatory should be inspecting.


I expect that half of the comments will be folks talking about how stupid this is, how stupid I am for doing it, and suggesting all sorts of theories as to why that is. Again, this is just one man’s experience and opinion. Don’t do it if you don’t want to (don’t do it if you do want to). As much as I appreciate hearing theories from folks that haven’t done it, haven’t fully thought it through, dismiss it outright because it’s “wrong,” or otherwise intuit or “feel” various reasons as to why it’ll break your gun and explode in your face, etc, they’re just theories. In practice, this works on all levels.

Again, I’m not suggesting anybody actually do this. You should probably also never shoot .223 in your 5.56 chamber, 7.62×51 in your .308 chamber, 2.5″ shells in your 3″ chamber, .22 Short in your .22 LR chamber, etc etc etc ad nauseam. (yes, I realize these all headspace off the shoulder or rim)

Random, Unverified “Historical” Note

Well, the verifiable part of this history is that the Soviet military specifically designed and required that its ammunition was incompatible with NATO firearms, meaning no captured ammo could be used against them. The sketchy part of the history is that this doesn’t mean Soviet firearms weren’t made to also run on NATO (or NATO-country) ammunition. Many folks claim that at least the Makarov, which was chambered in 9x18mm Makarov, was designed to reliably and safely cycle .380 ACP (9x17mm) and can also safely fire 9×19 (9mm Luger).

The larger diameter — 0.365″ vs. 0.355″ — Makarov bullet can’t be chambered in a .380 or in a 9mm, but the .380 round chambers just fine in a Makarov pistol. While .380 will actually feed from the Mak magazine, fire, and eject reliably, 9×19 has to be dropped into the chamber one at a time. It also exhibits excessive headspace, as you’d expect, but likely thanks to how much gas and pressure blows by the too-small projectile (it doesn’t seal in the bore), it doesn’t blow out cases and doesn’t seem to beat up the pistol.

While there are myriad YouTube videos and forum posts confirming reliability from .380 and successful function of 9mm (even on aluminum-framed, cheap-o, reproduction Makarovs), it’s probably best left to SHTF use only as either of these instances are more “extreme” than running .40 cal in a 10mm due to significantly larger differences between the rounds vs. the intended cartridge.


  1. avatar Texheim says:

    You’re stupid. There, self fulfilling prophecy.

    1. avatar joe says:

      Not a completely terrible idea. Now it’s a self fulfilling prophecy.

    2. avatar DJ in the mountains says:

      Only time a comment is STUPID!!!, is when it lacks definition.

      saying your STUPID without any reason why you believe this is self fulfilling prophecy of stupidity.

      Glock G40 here, I shot .40 out of it for a long time before I shot 10mm, I like to practice with the .40 and when in the woods YOU bet I have hot reloads for the berry patch 🙂

      just like my .357 and .38 and .44 mag and .44 special…

      never thought twice about using the short case for practice… and I have shot and fired my Ruger Redhwawk .44 and gp100 .357 more than 10k rounds each…

      thanks for the article.

    3. avatar Conrad Heinz says:

      Stupid are the people staing at empty shelves because obama has banned 10 mm as a

      ‘Mankiller” bullet too dangerous for the Fful posting IIIIBI! THanks fo this thoughtul posting I will grab 40.sw

      1. avatar Gun-toting Liberal says:

        Can anybody provide factual literature showing where Obama has banned specific firearms or ammunition? I forget, do we vote for things or does our dictator make decisions for us? Oh, wait, we don’t have a dictator.

        1. avatar Jeremy S. says:

          10mm has absolutely not been regulated in any new way whatsoever. It’s just normal handgun ammo and has never ever come up specifically in any legislative discussion that I’ve been aware of and I don’t see any reason it would be. It’s still considered a somewhat “niche” caliber and it’s standard handgun ammo — less powerful than many handgun calibers that also aren’t on anyone’s radar for any sort of “ban” or restriction.

          The administration did pass some laws stopping the import of certain Russian ammo, I do believe, especially 7N6 (steel core 5.45×39).

        2. avatar BbHrdZ says:

          there is none. the ATF handles these issues, including the 5.45 x 39 ammo issue. a court just ruled that the ammo is illegal. but the group that brought the lawsuit really made a poor case. just based on the ATF rule that a round has to be .22 cal or larger and capable of armor piercing, would have won the case. a 5.45 is smaller than .22 cal.

          the other thing, there is not a deterministic definition of armor piercing, type of armor, velocity, mass, nothing to actually measure (well, .22 cal is measurable). the 5.45 debacle was base on the rounds ability to be fired from some type of hand gun (you could design a .50 BMG pistol), that was not specified by any brand. a short bbl version of this 5.45 firearm, would probably not actually yield the velocities need to achieve armor piercing.

        3. avatar Jeremy S. says:

          The 5.45×39 round is larger than .22 caliber.

        4. avatar Andrew Lews says:

          @Jeremy S
          Umm sorry dude, but math says 5.45mm is .215in

  2. avatar Another Robert says:

    Re the “historical” note: I used to be on a P-64 forum, one of the members there averred that he regularly fired .380s in his CZAK with no issues. I haven’t tried it, especially since 9×18 was about $5 less per box anyway. Like the author says, if SHTF and 9×18 disappears, maybe…

    1. avatar arron says:

      Slightly off topic, but relevant to the final historical note… I cant speak for ammunition compatibility on NATO/Soviet small arms, as I have no personal experience on that issue. what I do have experience on is firing 81mm NATO mortar rounds through the Soviet 82mm Mortar. it works, and it actually works fairly well. A learning point on that is the 81mm round goes significantly further with more bang for your buck than the Soviet 82mm mortar ammunition, though that may have been partially due to the age and condition of some of that 82mm ammunition.

      1. avatar Hayabusa says:

        I remember being told when I was in the Army that the Soviets designed their mortar with an 82mm bore for specifically that reason: so that they would be able to fire any captured stocks of NATO 81mm mortar ammo through their own mortars, but NATO troops would not be able to do the same with captured 82mm ammo.

        I have no idea if this is true or not, but I think a lot of Cold War-era veterans have probably heard the same story.

        1. avatar arron says:

          we had heard the same thing, and after some research to make sure we wouldn’t blow ourselves up decided to use some while we were training partnered forces. Good learning experience all around.

        2. avatar William says:

          I know this is an old comment, but I wanted to correct this misconception. 82mm had been set as a Soviet mortar caliber before WW2, the same as 100mm, 122mm and 122mm as artillery calibers. If they designed it with foreign ammunition I’m mind, it was the 81mm (labeled 8cm) German mortars, not Western European. An instance where guns were designed with this in mind was WWI German field artillery, which was 77mm. The idea was that captured pieces could be bored out for German ammo but the reverse could not happen (French field guns were 75mm, English and Russian 76.2mm).

  3. avatar SteveInCO says:

    I’d love to find some 1200 FPS practice ammo. Not so anemic as to be 40 S&W in a longer case, but not so stout that I’d be wondering if several thousand rounds of it might break the gun,

    And it seems like most defensive loads are either anemic (i.e., might as well be .40), or actually designed to penetrate three feet for bears rather than for people.

    Given what’s actually offered on market, 10mm is rapidly becoming pointless outside of bear preparedness.

    1. avatar Jeremy S says:

      Armscor 10mm is rated at 1,050 fps and is also one of the more affordable loads out there. Actually, the lead photo is an Armscor 10mm and an Armscor .40 S&W…

      1. avatar SteveInCO says:

        Maybe I’ll check it out. But note the range given in the post: 975 (might as well shoot 40) to 1300 (full house), a difference of 325 feet per second. This round at 1050 is only 75 footpounds into that range, between a fifth and a quarter of the full house. It’s only an attached garage load. I want something that’s most of the way to full house without really pushing the envelope, so that the gun behaves like a 10mm during practice.

        1. avatar SteveInCO says:

          *75 feet per second. I guess I have too much blood in my caffeine stream still.

        2. avatar Jeremy S says:

          LOL, sorry. You said 1,200 and for some reason I obviously saw Armscor’s 1,050 stated but was thinking in my mind that it was 1,150. Sig Sauer Elite FMJ (and JHP) is rated for 1,250. Remington UMC is 1,150 but I’m not sure they even currently make that load. I think one of the issues at this point is that the big brand names are usually even more expensive than Underwood, and Underwood is awesome stuff. They do a reduced power (for them… it’s like 1,250 fps) load for the Colt Delta Elite but it’s only in JHP so it’s more expensive. A light bullet should beat the gun up less even if it’s going really fast, and Underwood sometimes has 135, 155, and 165 grain 10mm loads.

          If you’re shooting it through a 1911 or something, yeah, I’d probably be concerned about running tons of full-house loads through it also. If it’s a GLOCK, go nuts. The capacity and the ability to run the most powerful 10mm on the market without concern are the main reasons I went G20 instead of something else. I don’t really like Glocks in general, but in the case of a 10mm that’s for more than just target shooting it was the clear choice to me…

      2. avatar RandallOfLegend says:

        I chronograph 180 grain 40 s&w federal at 1025 fps out of a 3.5 inch barrel. A 10 mm should clock 1200 at that weight.

        1. avatar Russ says:


          If you clock Federal at 1,025 fps in .40, then that is what it will be in the 10mm. Federal loads them on the same table, same powder. They used to list them at 1,000 fps each. I called up their technical department one day and discussed it with them. They said most people don’t want the recoil of full 10mm 🙂 Anyway, Lo’ and Behold within a month the new listing for it was 1,030 fps, which means the only thing they changed was their website.

          If you get a chance to check it, post it here. Has anybody else chrono’ed the .40 and 10mm Federal?

    2. avatar John Rawlings says:

      I bought some re manufactured 10 mm from a Georgia Arms vendor that consisted of 170 grain bullets advertised at 1200 fps. It was $31 for a 50 round bag. More potent than a .40 S&W load for sure, but not loaded quite as hot as brands such as Buffalo Bore, Ted Nugent Ammo, or Double Tap.

      As for economy, if you intend to shoot real full strength 10mm loads frequently it would be worth the trouble to set for reloading.

  4. avatar Tom in Oregon says:

    I thought this was common knowledge.
    It’s cheaper practice ammo…
    I’ve never had a problem doing this.

  5. avatar Sc says:

    The article mentions that standard glock barrels have less-than-complete chamber support. However aftermarket barrels have much more complete chamber support. If that’s the case, is there more of a danger putting .40 through an aftermarket barrel than there is putting it through a standard glock barrel?

    1. avatar Happy Camel says:

      No extra danger, however in the case of .40 in 10mm, not much more appreciable saftey over stock. So no matter the configuration, .40 in 10mm is good to go (in glock).

      1. avatar Jeremy S says:

        ^^^ what he said. An aftermarket barrel with better chamber support mostly negates what I feel is the safety benefit from shooting .40 in a 10mm barrel that’s mentioned in item #1 in the post, but you still have #2 going for you. I’ve shot a hundred-ish .40’s through my 10mm Lone Wolf barrel w/out issue, but the vast majority of them went through the factory barrel.

        I didn’t mention in the post that I’m certainly not against conversion barrels. Drop a .40 conversion barrel into your 10mm gun and you’ll see a small bump in accuracy vs. running .40 in the 10 barrel, but the reason I often run .40 through my G20 in the first place is because it’s a lot less expensive…so spending $110+ on a conversion barrel means shooting a hell of a lot of rounds before breaking even on that investment. Since the .40 runs 100% a-okay in the factory G20 anyway, I’m not spending that money on a barrel I don’t want or need…

        1. avatar Sc says:

          The reason I asked was because I am very interested in picking up a g29sf for bear defense, but also for regular ccw. I already have an xd in 40 that my wife has been coveting. So i figured I give her the xd, and pick up the g29sf.
          I want to be able to put real full bore 10mm through it so I need an aftermarket barrel with full chamber support, plus if I want to carry it during hunting season I need a greater than 4 in barrel to stay legal in my state. Before i learned I could run 40s through the stock barrel I had planned on having to buy a 40 conversion barrel to save on ammo costs and a complete chambered 4+in 10mm barrel. Now I can skip the 40 conversion and only pick up the longer barrel. I’m sure I can get decent defensive 10mm loads (effectively 40 sw in a 10mm shell) for ccw with the stock barrel and then some full bores to run in the longer barrel for bear defense and hunting season in general. Running the stock config with the weaker 10mm’s won’t be much different than my xd40, and I can up the ante with the long barrel when necessary and run 40s through it to practice with either barrel. Now I have to put my 264lbc ar build on hold so I can pull together the cash for the 29sf.

        2. avatar Jeremy S says:

          Although I agree with absolutely everything you’ve said, I’m not going to admit it because nobody should ever run the wrong caliber through their firearm, of course. Also, despite the fact that I have witnessed many thousands of rounds of .40 in a 10mm Glock w/out a single stoppage whatsoever, I still wouldn’t trust my life to it given the option of running .40-ish-strength 10mm instead, as you mentioned. So… yep.

          Also, with a G29 you can get magazine sleeves so you can run the 15-round G20 magazines while effectively extending the 29’s grip length. Easy to find those sleeves for Gen3’s, not entirely sure how many options there are for doing this on a Gen4. (fyi in case you didn’t realize, there’s no “SF” in Gen4. All of the 10mm and .45 ACP Gen4’s are SF-size and if you want to make the grip larger front-to-rear like the older non-SF models, you can add one of the included backstraps)

  6. avatar Nate says:

    You said you shouldn’t shoot 7.62×51 in a .308, but it’s the other way around. 308 is higher pressure.

    I’m definitely going to try some 40 through my glock 29. Thanks for this write up!

    1. avatar Jeremy S says:

      No the point of that was listing some things that are totally acceptable practice but are still technically running a different caliber than what the gun was “made for.” IMHO, .40 in a 10mm GLOCK is also totally acceptable on the exact same level as running the lower-pressure 7.62×51 in a .308 chamber or a .22 Short in a .22 LR chamber, but it isn’t seen that way. Most people see this and consider it to be stupid and crazy and a hand grenade waiting to happen, etc etc…so the snarky point was that if you think that of the totally-safe .40 in a 10mm Glock use then you should also apply it to other “wrong caliber” use as well, including that list of ones absolutely everybody does because they’re also completely safe.

      1. avatar Nate says:

        ah- yes. I stand corrected.

  7. avatar PeterK says:

    Yeah, I have thought this was the case. I don’t possess a G20, though. :p

  8. avatar mdc says:

    I like the 165 jhp 1400 fps full power for carry 2 legged critters. Sweet spot for social work in 10mm. Late fall/winter for me.

  9. avatar Brandon says:

    I’ve got a Coonan 357. I can shoot 38s out of it all day but I do have to change out the recoil spring for the lighter one that comes with it. It has zero problems doing this and was designed to do this by Dan Coonan himself. I’ve also got a g20 Gen 4. I can see absolutely no difference in this.

    1. avatar int19h says:

      There is a huge difference, because .357 and .38 headspace on the rim – so the length of the cartridge doesn’t really matter so long as it can fit in the chamber.

      1. avatar Jeremy S says:

        Yeah I don’t think it’s a huge difference at all. The extractor holds the round in place just like a moon clip on a revolver. For the record, there are multiple revolvers made for 10mm that are obviously fully approved by the manufacturer for shooting .40 through. Same moon clips are used for either caliber. In the case of a 10mm GLOCK, the 10mm headspaces off the case mouth and the .40 S&W is held in place by the extractor, and the effect is entirely identical to running a shorter round in a revolver, whether that’s through the use of a moon clip or it’s a rimmed round…

        1. avatar jim c says:

          The moon clips used in revolvers are used to mimic the case rim like those on a .357/.38. Hardly the same as trusting an extractor to set or keep consistant headspace. The extractor is not designed or intended to perform this task. It is foolish to think at some point it will not fail to do its unintended job and possibility cause a catastrophic failure to the host gun. If you want to shoot .40, why not just buy a .40 instead of unnecessarily risking a failure? Glocks are cheap compared to other 10mm handguns on the market. I see no reason to use an autoloading cartridge in any other chamber other than the one its designed for. Why risk it when the conversion parts are available? Just because you fired 5000 rounds safely didn’t mean 5001 won’t blow up and cause irreparable damage. Yes sir, I saved 10 dollars per 50 rounds by shooting .40 out of my 10mm. All it cost me was my eyesight and three fingers on my right hand. No thanks. My G40 will eat 10mm, I’ll feed the .40 to my G22.

        2. avatar Jeremy S. says:


          1) the job for which the extractor is designed to do is significantly harder than just holding a round against the breech face. This task would account for a teeny, tiny, minuscule fraction of its normal workload. And as mentioned in the write-up, the extractor does this anyway way more often than you might realize. It is common for rounds to not contact the end of the chamber. Brass can be on the short end of the acceptable length variance and not headspace in a given chamber but, instead, be held by the extractor. I’m sure both of your Glocks have done this many, many times. In that regard, it actually is an intended job of the extractor.

          2) Extractors break all the time. It’s one of the most common parts failures in pistols. They don’t result in catastrophic failure. They result in a broken extractor.

          3) I still contend that .40 in a 10mm Glock is safer than .40 in a .40 or 10 in a 10. Again, this is because it fully chambers every single time, and case support is demonstrably better than with the correct caliber in the correct barrel. This negates the #1 cause of Glock “kabooms,” which occur in .40 and 10mm models far more than any other.

  10. avatar Geoff PR says:

    “….22 Short in your .22 LR chamber…”


    I did that twice!

    First time, it blew my hand clean off!

    Second time, it blew it right back on!


    1. avatar CarlosT says:

      But backwards! Oh the humanity!

      1. avatar Geoff PR says:

        It makes it easier to give yourself the finger…

  11. avatar Gary McClenny says:

    On the historical side, the .40 S&W is just the 10 mm come full circle to what Jeff Cooper wanted Bren to build in the first place. It is essentially a .41 AE rebranded. Had the ten been built the way it was specified the 40 would have never been built.

  12. avatar derek says:

    kind of a shame one of the ammo companies don’t take advantage of the rounds flexibility and offer a full power(1300fps) , medium(1150fps) and then a lite(1000fps) loads. but I guess if you were really into it you could reload to your own specs.

    1. avatar Jeremy S says:

      One of them loads their JHP a couple hundred fps faster than their FMJ, but I’m blanking on who that is. Might be PMC or might be Sig Sauer w/ their new line…

    2. avatar Accur81 says:

      Underwood and Buffalo Bore are hot, many 10mm defensive loads are like a warm .40 like Federal, and there even mid level 10mm in the 1200 FPS range. All factory. Enjoy.

      1. avatar BlueBronco says:

        Ironically, the Winchester 175 gr Silvertip HP is much hotter than the average ammo from Federal, Rem, etc. I guess they were trying to get over the anemic reputation of the 9mm silvertip ammo of the 80s.

  13. avatar Joseph says:

    I carry a .40 because I own one, I’d carry a 10 if I owned one. For me it’s that simple but I’d carry a .380 if I owned one, in the end it’s more about the weapon you have to defend yourself with then what it is… Use what you have right there with you… But, that’s just me..

  14. avatar TomE says:

    .40 S&W in GLOCK brand GLOCK 10mm… What can it hurt?


    1. avatar Jeremy S says:

      That link has literally nothing to do with running .40 in a 10, and I reaffirm my position that doing so is SAFER because it mitigates all of the causes of Glock kabooms mentioned in that link. It improves chamber support, reduces the risk of out-of-battery ignition significantly if not entirely, and gives you a thicker chamber, barrel, and slide to better cope with any rounds that are over-pressure for whatever reason.

      1. avatar TomE says:

        I encourage the readers to read the link. GLOCK has a history inadequate chamber support in high pressure 10mm caliber barrels. GLOCK recommends use of only designated, jacketed, factory produced ammo – and in some circumstances even when these dictates were followed kB!s were the result.

        1. avatar Jeremy S says:

          Exactly. That’s why the .40 S&W, which sits deeper in the 10mm chamber, is safer. As mentioned in my write-up here, the proof is in the fired brass, which lacks the “Glock smile” that’s always seen in .40 S&W or 10mm when shot through the correct barrels due exactly to the relatively poor chamber support that you’re talking about. The cases bulge into the unsupported part of the chamber (right over the feed ramp), causing that “smile.” When you shoot .40 in a 10, it isn’t there because the case walls are deeper into the chamber where it’s fully supported.

        2. avatar TomE says:

          .40 S&W am 10 mm Auto seat on the case mouth, so ‘.40 seats deeper’ ? Making it safer and giving better case support… I find it hard to believe extractor and firing pin function are unchanged. Anyway, I really hope I don’t see you in the the next case study on GLOCK kB!

        3. avatar Jeremy S says:

          .40 in a 10mm chamber does not seat on the case mouth.

        4. avatar TomE says:

          .40 S&W seats on the case mouth in a .40 chamber. You’re counting on .40 to seat on the extractor hook in a 10mm chamber, which is less than ideal to say the least. You’re not doing firearms safety or the readers of this blog any favors by posting and defending your trash thinking.

        5. avatar Jeremy S says:

          I don’t suggest anyone does it themselves. You should never run the wrong caliber in a firearm.

          However, I object to the claim that this is “trash thinking.” I’m offering real life experience and demonstrable evidence — such as a near-complete elimination of case bulge — that your theoretical objections (i.e. “thinking” without a basis in testing or experience) don’t actually pan out.

      2. avatar TomE says:

        You’re making a case and not suggesting anyone do it, but just saying… and defending your choice.

        GLOCK Fanboi: “I load my daily carry GLOCK brand GLOCK, with rainbows and it shoots unicorns everytime, forever. It’s Perfect, Gaston told me so. It’s also safe to carry with my 1.5lb Ghost Rocket trigger ‘cuz my safety is right here [demonstrates hooked index finger next to face like Eric Bana to the Ranger Col. in ‘Blackhawk Down].”

        I’ll stick to GLOCK brand GLOCKS in 9mm as Gaston intended.


        1. avatar Jeremy S says:

          Well half the reason I posted this is because I enjoy arguing in the comments haha. Defending it is the reward for writing it 😉

          FYI, it’s the only Glock I own and I have always disliked Glocks. I do shoot them well, but I don’t like the ergos of the grips, the grip angle, the feel of the plastic they use, the appearance of the square slide particularly from behind the gun, the triggers, the trigger safety blades, the plastic-covered magazines or their annoying baseplates… I’m not exactly the archetype of a Glock fanboy. But… when it came to a 10mm for woods carry (and I chose the caliber first), it was simply the best choice based on capacity, ability to shoot full-house loads without concern for durability, and cost. So I got it despite the fact that it’s a Glock. And, yeah, I like it a lot.

  15. avatar Accur81 says:

    I agree with you, and your technical explanations make it clear that you and your friends are making informed decisions. Shooting the Bull is certainly shooting multiple calibers out of his Glock 21.

    I’ve had such fun with my .40–> 9mm Glock Lone Wolf conversion barrel that I could definitely see a Glock 29 in my future. And if I screw up an extractor in the G23 shooting lots of 9mm rounds (with the conversion barrel and upgraded recoil spring and guide Rod) I can always replace the extractor. I’m about 1,500 rounds in, and everything is working fine. I’ve run 115, 124, and 147 grain standard pressure and 124 grain +P HST and Underwood with no issues.

  16. avatar Detroiter says:

    I always suspected this to be the case. Still not a glock fan, and still seriously wondering about eaa and their durability with 10mm full power loads. Need to find one to look at the firing pin….not that I’d ever actually shoot 40 through a 10mm…….

  17. avatar Curtis in IL says:

    Why not just load 10mm cases to .40 pressure?

    1. avatar Jeremy S says:

      Well I don’t reload brass stuff (only shotgun shells), but if I did I’d just load the 10mm to 10mm pressure. I like the stronger round, and only shoot .40 because it’s cheaper. If you do reload, .40 brass is also cheaper than 10mm brass and small pistol primers are often cheaper than large pistol primers, so there’s that… plus I still contend that the shorter case length is actually safer in a factory Glock barrel due to achieving better case support and lower likelihood of excessive headspace (slightly out of battery ignition)…

      1. avatar Curtis in IL says:

        Just food for thought, Jeremy – There’s more savings in reloading pistol rounds than shotgun shells, assuming the brass is free (because it’s the most expensive part of the cartridge to begin with). If you’re buying factory ammo now, save your own brass (and everyone else’s) and start reloading when you have a decent inventory.

        1. avatar Jeremy S says:

          10mm is the round that would get me to kick off reloading brass cases for sure. But you don’t get that off the floor at the range. I’ve saved up all that I’ve shot and it’s more than enough now to start reloading it…it just isn’t a priority at the moment. But coming across free 10mm brass ain’t easy. It isn’t super common and everyone I’ve seen shoot it saves their own brass, too. Even if they don’t reload, because the once-fired stuff is still worth $$$ to those that do.

    2. avatar Anonymous says:

      Then he would get “glock smile.”

      On the one hand, he is firing 40S&W in a 10MM glock to eliminate the glock smile, but in doing so he reduces accuracy and uses a questionable method for which it was not designed.

      My solution would be… don’t buy a Glock in 40S&W or 10MM. I have an EAA witness, all steel, that fires 10MM in my 10MM barrel just fine, with no “smiles” or strange 3.6mm gaps between my cartridge mouth and a chamber rim intended to headspace a different cartridge.

      1. avatar Jeremy S says:

        That’s not why I’m doing it. I’m doing it because .40 S&W is cheaper to shoot and I don’t own a .40 S&W Glock so I run it through my 10mm Glock. Accuracy is hurt, but it took a sandbag rest and a laser for me to verify it. It’s not a difference that can be measured if you’re shooting offhand and aren’t a bullseye competitor

  18. avatar Dale says:

    Everyone can have their opinion. When I see some scientific data from some rigorous testing then and only then will there be a point to this.

    1. avatar Jeremy S says:

      IMHO, it isn’t an opinion when you’ve seen multiple GLOCKs do this for 10,000++++ rounds each (aka “data”) without a hitch or side effect of any sort, and from an engineering perspective there is simply no reason why it should cause any hitch of any sort. Granted, my professional engineering experience is only related to car parts R&D, but turbochargers, cylinder heads, exhaust parts, suspension components including shock/strut valving, etc, involve plenty of variables including friction, heat, pressure, moving parts, metallurgy, coatings… I’m 100% confident here but nobody should ever shoot .40 in a 10mm Glock anyway.

  19. avatar Thomas Martens says:

    I’ve been doing this for years. Even shot competitions with .40 instead of 10mm through my G20. Of the thousands of rounds I’ve fired through it, 1/3 have been .40s. The people who say not to, really don’t know what they are talking about.

  20. avatar Former Water Walker says:

    Makes sense to me. I shoot 40 very well and if I EVER get a 10mm Glock this is good to know.

  21. avatar Anonymous says:

    Obviously the bullet has a few more millimeters of free bore travel before it contacts rifling than a 10mm will, but this is far from unique in the firearms world.

    I would be concerned about 40S&W ammo bullets shaving against the rim where the 10MM auto case mouth headspaces. But I haven’t tried this in practice.

    Other than “headspacing off the extractor” instead of the case mouth, everything else is functionally the same as 10mm as far as a GLOCK is concerned.

    They are completely different in how they are being suspended in the chamber – other than that I guess they are functionally the same.

    Headspace. “Kabooms” can occur due to excessive headspace — the distance between the breech face and the end of the chamber (the lip on which the case mouth stops) — caused by the cartridge protruding too far out the back (ejection port side) of the chamber…


    Since these rimless cartridges are designed to headspace off the mouth of the case when it hits the front of the chamber, a piece of brass that’s slightly too long, a bullet that isn’t seated deeply enough, heavy chamber fouling, and/or a slide that doesn’t quite go fully into battery can and does cause this dangerous scenario.

    I think you mean too little headspace. Excess headspace would be extra space between the case mouth rim inside the chamber and the slide/bolt face. (Which is exactly what you are doing when using a 40S&W in a 10MM chamber) The excess headspace allows the brass to expand around the detonation resulting in a thin area on the brass which can cause case-head separation:


    Excessive headspace is “extra space” not the lack thereof. You are concerned with too little headspace – which is where the cartridge is too long and the fit is too tight where you are concerned that the case head will extend beyond the chamber against the bolt/slide face. Too little headspace “can” result in your firearm blowing up in your face if you crush the case mouth (like a tapered case mouth) into the throat of your bore. Too much headspace (excess headspace) may result in case head separations and primers protruding from their pockets. If they protrude too much, it can cause erosion on the bolt face.

    1. avatar Jeremy S says:

      No. That diagram is misleading. Headspace is measured as the distance from whatever stops the cartridge’s forwards movement to the bolt/breech face. In the case of a straight-walled pistol cartridge, that means from the chamber lip to the breech face. It is never a measurement of gap between breech face and cartridge base. The phrasing here gets complicated, because when I say Glock Kb’s can happen due to excessive headspace (meaning the distance from chamber lip to breech face is too long), I don’t actually mean the firearm’s designed headspacing is incorrect. I mean that the slide isn’t going completely into battery due to a case that’s too long or for another reason, which effectively extends the headspacing measurement. Case walls end up without enough support and they blow out. Your diagram can also be an instance of excessive headspace, but instead of extra space out the back side (breech side), the extra space is actually on the front. The chamber is reamed too deeply. That’s what makes that rifle round drop in too far and the gap appear between breech face and cartridge base. In that diagram, the extractor is not capable of holding the rifle round up against the breech face like a Glock’s extractor is. Additionally, a rifle round at 65,000 PSI has completely different rules from a pistol round at 35,000 PSI. A reasonable gap between cartridge base and breech face is really a non-issue in a pistol. Revolvers must have a gap between case base and recoil shield or the cylinder wouldn’t turn. At any rate, slide a .40 S&W cartridge up the breech face of a 10mm slide and you’ll see that it’s held completely snugly against the breech face by the extractor. The extractor is actually pulled out by the rim of the cartridge and is clamping down on it under spring tension. There is no gap at all.

      ^^^ I didn’t fully address the fear of a breech face-cartridge base gap in the post like I did in the comment here, so I’m going to add a couple photos to the extractor section now. Thanks.

      1. avatar Anonymous says:

        I don’t actually mean the firearm’s designed headspacing is incorrect. I mean that the slide isn’t going completely into battery due to a case that’s too long or for another reason, which effectively extends the headspacing measurement.

        Yes – but that is not called “excessive headspace.” The glock smiles aren’t a result of excess headspace – they are a result of improper case head support which causes the bulge.

        You said:
        “Kabooms” can occur due to excessive headspace — the distance between the breech face and the end of the chamber (the lip on which the case mouth stops) — caused by the cartridge protruding too far out the back (ejection port side) of the chamber…

        Which led me to believe in your statement that “the cartridge protruding too far out the back of the chamber” equates to “excessive headspace.” Which it does not. Like you said (and I agree with) the cartridge from the bolt/slide/breech face to the chamber lip where the cartridge mouth stops against the chamber is the “headspace.”

        Too little headspace will result in the cartridge case head protruding (breechface side) too far from the chamber. On a bolt gun, the bolt may not go fully into battery and the locking lugs might not fully close in place. On a slide/pistol the slide will be slightly pushed back and the slide/barrel lockup might not have occurred. In this case a “kaboom” would result in a great deal of damage/injury/death.

        Typically if there is excessive headspace, The firing pin may not detonate the primer as it may have been too soft a hit. If it does detonate you may get gassing around the cartridge, around the primer, and a puff of gas hitting you in the face – but no injury (as long as you are wearing glasses) and minimal damage.

        The glock issue isn’t a result of either, but of too little cartridge case head support which is a design problem – one you have overcome by using a shorter overall cartridge length.

        1. avatar Tommy says:

          Anonymous is correct. Jeremy should not be writing articles recommending dangerous practices when he doesn’t even know the importance of headspace and how kabooms from too much headspace happen. Your terminology of “too much” and “too little” headspace switched around.

          When you have too much headspace which is the situation here, the case can slide down the chamber. When it goes off, the shoulder/mouth expand and grip the chamber, but the rear of the case being thicker does not expand and is free to move back with the internal pressure stretching the case. If you have brittle brass, or fatigued brass (from reloads), the amount of stretch could rupture the case and you have case head separation. KB!

          Too little headspace usually is not the cause of kabooms as the article insinuates. Most guns won’t let you fire if the bolt is not fully into battery.

  22. avatar Anonymous says:

    This whole article brings one point to mind more than any other…


    Google it.
    Click on images.

    1. avatar Jeremy S says:

      Is that anything like Sharknado?

      Thankfully, .40 in a 10 reduces the chances of Glocknado attack (my $0.02)

  23. avatar Will says:

    The elimination of the “smile” in fired brass is an interesting result. In theory, since the back of the cartridge is the same, it should look the same after firing.

    I’m thinking that it is a side effect of not being captured/restrained between the breechface and the chamber end during ignition. What appears to cause the “smile” is the round gets shoved/bounced to the top of the chamber while the barrel is pulled downward slightly by the locking lug angle. I measure a sloppy fit of .011″ between the case dia and the chamber mouth dia of a stock G27. If the round is allowed to float in the chamber when fired, it should show less of a “smile”, or none at all.

    This potential gap is visible if you take the slide with barrel and slide a cartridge under the extractor and then push the barrel over the round. Push the barrel all the way up, and then pull it down a little bit. You can see the action of how the gap is always going to have a vertical displacement due to how the barrel and cartridge rim are constrained in side to side movement.

    I’m unsure if it is the brass floating during the ignition process, or that the chambering process leaves it more centered without the constraint of hitting the chamber end. Could even be a combination of the two.
    Cutting a .40 case a bit short and shooting it in a .40 may show the loss of the “smile”, for verification of this. DON’T set the bullet back, keep the same OAL to avoid a k’boom.

    1. avatar Jeremy S says:

      “What appears to cause the “smile” is the round gets shoved/bounced to the top of the chamber while the barrel is pulled downward slightly by the locking lug angle.”

      By the time the barrel is pulled down and unlocking, the bullet is supposed to be out of the barrel and the pressure way down. It’s the design intention of a browning-style, recoil-operated pistol not to unlock until the pressure is dropped. At that point, it should be too low to cause the case to expand like that. Glocks are actually sprung kinda soft, but the bullet is still usually out of the barrel before the slide is even moving rearwards at all. Like in this pic, the bullet is a few inches out and the slide is like one mm back. It’s still a good ways from unlocking…

      I think it happens when the pressure is high and everything is still completely locked up, and it’s due to a Glock chamber having less case support than basically any other pistol’s chamber. Actually, this is all pretty well documented. The smile comes from the half-moon shaped (smile-shaped) gap in case support at the top of the feed ramp. For instance, HERE’s a photo of an aftermarket Glock barrel vs. a factory Glock barrel, and you can easily see where and why the smile happens. The case walls are thin, and it expands into that unsupported section.

      .40 in a 10mm barrel generally chambers more deeply, so that gap in support is reduced.

      1. avatar Will says:

        “.40 in a 10mm barrel generally chambers more deeply, so that gap in support is reduced.”

        This is not possible, unless the extractor claw is very loose in some manner. In fact, it would not fire, due to inadequate firing pin engagement, if it was far enough in to eliminate the smile from being formed at all.
        You can verify this by doing the slide/barrel check as I mentioned. You should find there is little, if any gap between the brass base and the breechface, and you will not be able to force it away from it. Might be simpler to check this without the barrel in place, as you are really checking extractor tension, to some extent.

        I think you misunderstood what I meant when I talked about letting the cartridge “float” during firing. I should have been more clear in my statement. —[ Just realized that my thoughts weren’t quite finalized on the engineering of the situation, as there is a variable I don’t have nailed down, so some fuzzy thinking got expressed]— I was referring to the fact that it isn’t being forced to a specific location/orientation at the end of the loading/chambering cycle. It has some range of movement that would allow it to theoretically end up stopping anywhere within that ~0.011″ vertical chamber slop. But, something in the dynamics involved would seem to cause it to consistently end up at a different location than it does when fired in the correct length chamber.

        I was initially thinking that the smile was formed due to the brass being located at the top of the chamber slop, but now I’m thinking it might actually be the reverse. I was envisioning the brass ballooning down in the gap, and continuing to flow over the edge of the ramp. However, it may be that if located at the bottom, the brass would flow over the edge, while ballooning (expanding) to fill the rest of the chamber, which would not be noticeable to the casual examination.

        I’m not knowledgeable enough about brass cartridge expansion/release dimensions to make that determination. One would have to measure fired Glock brass to look for eccentricity of the body to base, to figure that out. It may require measuring body wall thickness relative to the location of the smile, to accurately determine it, since the unequal movement of the brass may not be reflected in the overall shape, but in flow within the brass material itself. This would have to be done with virgin brass, as previously fired brass would already have some sort of distortion introduced, making any conclusions drawn to be suspect. Neat! a new wrinkle in the Glock/brass story, it seems. I wonder if aluminum cases would show this situation more clearly?

        Being able to push the .40 cartridge to the top or bottom of the 10mm chamber, prior to firing, would go a long ways to figuring this out.

      2. avatar W says:

        I have been reloading 10MM brass from my Glock for a while now. I am up to 5 reloads on some of my cases. FWIW, I have not encountered any smiles on the spent brass. On the other hand, I don’t have a velocity fetish either. That is, I am not trying to set a Guinness record for most insane 10MM Auto load.

        The smiles that some encounter may be products of a few things. There is case support. However, there is also the web in the case. Further, there is the operating pressure. In addition, some pinheads appear to have dremeled (i.e.: “polished,” and I use that term very loosely) their chambers and ramps. Below looks dremeled, to my uneducated eyes anyway.


  24. avatar W says:

    Like JS, I tell people to specifically NOT fire 40 in a Glock 20. It’s not recommend, not designed, and may leave the weak of heart with conniptions, wet pants, or unable to leave their mother’s basements.

    I have done a few hundred rounds of 40 in my Glock 20 with no issues. I’ll continue doing so. The keys are the great extractor on the Glock which will hold the case very firmly agains the slide breechface, and the overbuilt slides and barrels.

    It’s too bad that Glock doesn’t come out with a statement saying that this is okay. They could popularize the 20 as a dual caliber, soft shooting pistol.

    Anyway, never ever shoot 40 in a Glock 20. And never use a conversion barrel, because Glock doesn’t authorize it. Never put a different connector in your Glock, Glock doesn’t approve doing so. Never reload ammo, because all manufacturers recommend against it. And, never run with scissors or a pistol.

    1. avatar Jeremy S says:

      LOL true. A few folks on YouTube commented on my .40-in-a-10mm Glock video saying this will void your warranty and you should use a conversion barrel instead. Apparently they missed the part where that also voids your warranty.

  25. avatar d.rap says:

    seems like you hear how powerful that 10 mm is and here we hear it’s nearly the same… have to check that out.

    1. avatar Jeremy S says:

      Some is, plenty isn’t. If you want full-power 10mm — the stuff that’s loaded to the full (yet safe) potential of the cartridge — you’ll want to shop with Underwood, Buffalo Bore, and DoubleTap, and you’ll end up with ammo that’s like 50% hotter (in energy level) than the hottest .40 S&W.

  26. avatar Rand says:

    From a liability standpoint, I can’t believe this article even got published.

    1. avatar Jeremy S says:

      Your comment is depressing. Such is the state of tort law in ‘murica, I suppose. I wonder if I could win a lawsuit against you for depressing me with your reminder of this reality.

      You should never do anything the owner’s manual tells you not to, most certainly including shooting the wrong caliber in a gun. This isn’t a Bloomberg-owned website, so we won’t mandate your soda size, the kind of fat you consume, whether or not you should wear a helmet, or whether you should or shouldn’t shoot .40 in a 10mm Glock. I’m not telling you to do it. This is about why I choose to do it. I also drink rye whiskey and smoke the occasional cigar and SCUBA dive and even jumped out of a plane once. Feel free to do those things, too. Or don’t. Make up your own dang mind and own your own choices.

      “The purpose of education is to make the choices clear to people, not to make the choices for people.”
      — Peter McWilliams

      “Those who created this country chose freedom. With all of its dangers. And do you know the riskiest part of that choice they made? They actually believed that we could be trusted to make up our own minds in the whirl of differing ideas.”
      — Nat Hentoff

  27. avatar Greg says:

    Most of those arguments would also justify shooting 308 in a 30-06.

    Because sometimes the gene pool needs some filtering.

    1. avatar Jeremy S says:

      I don’t think any of the same ‘arguments’ apply. The only truly common thing between .308 and .30-06 is bullet diameter. If you think .308 is a shorter .30-06, you’re way off base. In fact, I think .308 is actually rated for higher pressures. And they’re necked cartridges so running a shorter case, even if it were 100% identical in every other way, is going to F the brass up and probably cause the case to seize in the chamber. The analogy of shooting .38 Special in a .357 is light years closer to what’s going on here.

  28. avatar AUGrad06 says:

    By this logic, a 380 should be able to fire out of a 9mm, correct?

    1. avatar Jeremy S says:

      .380 is not a shorter 9mm like .40 literally is to 10mm. They are different in many other ways. However, yeah, this has been done a gazillion times both intentionally and accidently and the worst result is that the pistol almost never cycles because of how much weaker .380 is.

  29. avatar TheOtherDavid says:

    Not sure if anyone is still following this discussion but – I confess. I drank the “it won’t blow up” Koolaid (Is that KaBoomAid?) and ran about 200 rounds of 40 cal FMJ and WWB JHP through my stock Glock 20 and 29. I didn’t have a single malfunction of any kind and running 40 through a beefy reinforced 10mm frame – the 20 especially – was a revelation. If someone had handed me the pistol and said “we put a 380 conversion barrel in it,” well, that’s what it felt like. I can see the appeal for competitive shooters, and the option of running a fine defensive load like Federal HST 40 in a 10mm frame (ending up with less recoil than 9mm) is intriguing. For me? today was mostly a SHTF check. Nice to know that when the zombies come I can run two good defensive calibers through these two pistols without modification. Will I make a habit of it? Probably not – I’ll continue to run 10mm through for training just to keep the recoil and OMG muzzle blast fresh in my mind. When I woods carry they’ll be loaded with 10mm, Hornady XTP 180 or the great Federal JSP hunting/defense load. But I get a warm fuzzy feeling knowing I’m good to go with 40 just in case.

  30. avatar NavyVet1959 says:

    Another option is to use .40SW brass to load 10mm power loads by loading to the same OAL as is used for 10mm. A bit less of the bullet is covered by the brass, but it works great.

  31. avatar Mark says:

    I see no problem using the 40 in a 10mm barrel. The my G20 chamber is longer then SAAMI spec. The trim to length is shorting then SAAMI spec. All factory ammo is made that I have measured is to right around the trim to length. None of the ammo I have for my 10mm touches the forward part of the chamber, it is all short.

    1. avatar Jeremy S says:

      Werd. Most folks simply don’t realize that their correct-caliber ammo is sometimes “headspacing” off the extractor instead of the case mouth anyway. Shooting .40 in a 10 guarantees it’s doing that every time, but shooting 10mm in a 10mm is far from any sort of assurance that it isn’t. The idea that this is some sort of a problem is ingrained in peoples’ heads but based on assumptions and supposition, not reality. In reality, it’s a total non-issue to the function, reliability, and lifespan of a Glock, a Glock extractor, or a Glock barrel.

  32. and this is the exact quote from the article when I knew for a fact the author has no idea what he is talking about. “The case is more fully chambered and better supported.” if in fact that was true and the case was deeper in the chamber, the firing pin would never reach the primer. Please don’t anyone do this, it is just stupid.

    1. avatar Jeremy S. says:

      Posted this below, but will copy and paste for you here as well. Note that it has nothing to do with the round being away from the breech face. In either case, the round is held firmly against the breech face with no gap at all. It’s how much of the round is being pushed out the back of the chamber due to the front of the case bottoming out. And it’s very, very easy to prove in actual practice, as stated below:

      It does sit deeper 99% of the time. In fact, .40 in a 10mm sits deeper in the chamber than .40 in a .40. This is precisely because it isn’t headspacing off of the case mouth, which obviously stops the round from going deeper into the chamber, but is being held in place by the extractor. This means the action is closing as far as it can physically close. The most obvious way to “prove” that this does, indeed, mean the round is deeper in the chamber is to fire a few rounds of .40 through a .40 cal Glock (e.g. G22) then fire a few rounds of .40 through a 10mm Glock (e.g. totally unmodified G20). You’ll notice that the case bulge — that “Glock smile” — is clearly shallower and smaller in the 10mm barrel. It isn’t a chamber support thing or a difference in the barrel, it’s the fact that the round goes deeper into the chamber. Firing 10mm in a 10mm barrel — even a .40 S&W power level load! — also produces a much larger “Glock smile” than firing .40, and this again is due to the case mouth bottoming out in the chamber and extending the rear of the cartridge out the back of the chamber a little bit farther.

  33. avatar voodoo says:

    Just buy a .40 cal barrel for your G29 when you want to shoot .40 and switch back to the 10mm for carry. And STFU already. All of you.

  34. avatar Don Keeton says:

    You said you probably shouldn’t shoot .223 in a 5.56 chamber. It’s the other way around. You shouldn’t shoot 5.56 in a .223.

    1. avatar USMC Heavy Weapons says:

      you are correct.

      obviously (in my opinion anyhow) he meant that sarcastically for the critics sayin u cannot shoot .40S&W in 10mm glocks.

    2. avatar Jeremy S. says:

      Nah, it’s correct as written. People say no .40 in a 10mm despite the fact that it’s a less powerful version of the same cartridge. Therefore, those same people shouldn’t shoot .223 in a 5.56 either or .38 in a .357. But they do, because those other two examples are commonly accepted. My point is that .40 in a 10mm Glock should be accepted as well.

  35. avatar Max says:

    Accuracy!!!…everyone talks about just that and some other things related!!..but MY QUESTION IS,what about POWER?..will the .40 shot through 10mm barrel lose power because of is shorter that the 10mm,what about penetration?..if less ,then..can we still use it for self or home defense?..thank you!!

    1. avatar Jeremy S. says:

      I touched on accuracy and it’s very close but just slightly not as good. Velocity is completely unaffected. Actually, as the barrel of a G20 is a bit longer than that of a G22, you can expect the .40 S&W from it to be a teeny bit faster.

      Even though I’ve run thousands of rounds of .40 this way and know people who have run many tens of thousands of rounds, all without one single stoppage of any sort related to the fact that it’s .40 in a 10, I still wouldn’t personally choose to do it for self defense use unless it was my only option for some reason.

  36. avatar Dave G says:

    Gee,I wondered about that firing pin issue, with the round sitting deeper. Do you need to use the .40 mags?

    1. avatar USMC Heavy Weapons says:

      Dave. I am not the author of the article, but no, the magazines are not interchangeable. I just load .40 S&W rounds into my Glock 20 10mm magazines, insert mag, cycle slide and plink away…

    2. avatar Jeremy S. says:

      The round is deeper in the chamber only because the chamber is deeper — it’s still held right up against the breech face by the spring tension on the extractor. The firing pin doesn’t have to travel any farther at all to reach the primer.

      The .45 ACP and 10mm Glocks have a larger frame with a larger magazine well. .40 S&W mags won’t fit in the 10mm frame. Well, they’ll fit but they’ll be loose and won’t lock in. The .40 simply goes in the 10mm mags. This actually isn’t even a weird thing, as pretty much every .40 cal 1911 (and there are a lot of them, particularly for competition) uses the exact same magazines as every 10mm 1911.

    3. avatar Jugchoke says:

      Personally, I dont believe that the round is ACTUALLY sitting any deaper!
      That is, not UNTIL the firing pin actually strikes the primer!

      At that point, ALL slop is removed in the length dimension as the cartridge is boosted forward, where it then expands out against the chamber walls.

      Until that point in time, the cartridge still isnt quite “tight and flat” against the bolt face, even being “supposedly” pulled back against the breech block by the extractor hook.

  37. avatar USMC Heavy Weapons says:

    For what its worth, in mid 40s now. When I was a kid, I learned handgun marksmanship on father’s .357 and mother’s .38 revolvers. Thus learned early in life about using .38 spc in Dad’s .357 magnum. My 1st civie handgun after USMC service was a Glock 17. Didnt care for the cartridge but loved the platform. Sold it when I came across a Gen3 G20C. Been pleased with it ever since and many yrs later still use it over my Glock 22 or even 23 for conceil carry. Around 2004 or 2005 at range (before buyin .40 S&W models), I decided to test the theory I had. I bought a box of .40 S&W and took the chance. Never regreted it since. Burned thru the the box, every round cycling perfectly and bought more, saving my 10mm defense rounds for more important use. Years later a dealer at local gunstore/indoor range freaked out and told me I couldnt use .40 S&W in my 10mm glock. I told him it has worked fine with over 2,000 .40 S&Ws down the barrel with no issues. He said I was lucky. Mentioned .38 spc in Dad’s .357 to him, showed him my range success for the day and insisted I know it is safe and will continue to use it for range time plinking…. Countless conversations (& some arguements) to this day and thousands more .40 rounds AND additional 10mm as well later, I dont need any convincing nor care if someone disapproves!! I KNOW .40 S&W works just fine in my Gen3 Glock 20C with 100% success rate. Never had to change a single part on the handgun. Never had a failure to load or eject. Only failures of any kind were the few dud rounds that are meaningless to the point at hand. Still havent met another Glock 20 user that does it but assume there are plenty out there…

    US Marine
    Seattle, WA area

  38. avatar S. McQueen says:

    Late to the debate.

    Jeremy is right: It works fine and one shouldn’t do it.

    This article was inspired by a competition shooter in 40 S&W Class. Other than that purpose (competition stock) ….Why would someone want to shoot 40 out of a G20 anyway?

    I get that it’s cheaper. But it’s not “practice” unless you are practicing to shoot 40 caliber.

    If you need to practice your 10mm skills in your G20, NO 40 cal is appropriate. (Neither are the watered-down 10mm commercial loads like Federal or UMC).

    If you want your 10mm G20 or G29 for woods or home defence or concealed carry, then you darn well Practice with Real, full house 10mm (Underwood, Double Tap, etc).

    Otherwise, you are Deluding yourself that you are becoming competent enough in operating your 10mm pistol.

    The pistol may be tthe same, but the entire experience is significantly different in 40 vs real 10mm in recoil, follow up, noise, muzzle-flash, adrenaline flow, etc., etc.).

    So, if you just want to shoot 40 out of a bigger or different gum – then have at it, that’s great, have fun.

    Otherwise this whole debate has discussed an interesting answer when there really was No Question asked first: A solution without a problem.

    Interesting though. Thanks.

    1. avatar Jeremy S. says:

      There are a lot of professional shooters and trainers who would disagree with your assertion entirely. They would suggest that solid fundamentals are more easily learned and maintained by shooting low-recoil calibers like .22 LR and by dry firing. Dry fire is huge to ingrain muscle memory and good fundamentals. .22 LR conversions (e.g. the Kadet Adapter) are popular for practice as well. Fundamentals tend to slip if you’re shooting a recoil beast all of the time. Less recoil is a great way to learn a new firearm and learn to shoot it consistently and properly before upping to the full power that it’s capable of, or in-between for practice sessions, etc. Plus the fact that it’s easier to shoot means you’ll probably practice more. And you can afford to practice more, too, as the ammo is cheaper.

      1. avatar CALVIN L NIKONT says:


        I have a Glock 20 and just bought 1,215 .40 brass and was thinking about either buying a .40 barrel of trying to find a revolver that would shoot both 10mm and .40. The S&W 610 would do just great but they are pretty rare and pricey. Ruger made a 10mm convertible to .38-40 but that too is out of production and they say that the .40 is on it’s way out with the advancements made in 9mm bullets.

        On the practice and recoil issue, I bought a .22 in ALL the different actions and having shot thousands and thousands of .22, I have eliminated recoil from my thinking. I shoot my Ruger Single-Six and then I shoot my .454 Casull and I don’t worry about what will happen after I pull the trigger. Like they say: If the recoil is going to kill you, there is no sense in flinching!

        One time I was shooting in the desert south of Barstow, CA with some friends and to end the day, as per our custom, I loaded up the 5 shot Freedom Arms model 83 Casull and shot last. Somebody must have short-stroked the hammer and advance the cylinder and extra hole. We were shooting at a clay pigeon on the hillside 80 yards away and when I pulled the trigger – nothing happened. I kept the barrel pointed at the target in case of a misfire and noticed how steady the barrel was. I cycled the hammer, pulled the trigger, and again nothing but if you had put a dime on the barrel it wouldn’t have fallen off. Same thing again and nothing. Finally it fired and I shot the clay pigeon that no one else had been able to hit.

        I drove my motorhome to the Hodge Road exit and drove west for a mile every weekend for over 3 years and went through brick after brick of .22 and finally numbed myself out to recoil. Same thing with my Winchester 9422 and my Marlin 1895 SS in 45-70. I’m very accurate with it because I’m not concerned about what happens after I pull the trigger. I also shot clay pigeons from that same hillside 80 yards away with my S&W model 640 2″ barrel .357 revolver. Unlike some guys who jumped out of the car, fired $100 worth of ammo and left, I spent more money on steaks and beer than I did on ammo. I arrived on Friday night and left on Monday morning and shot tons of .22. I tried to always go 6 for 6 on shotgun hulls. I kept shooting the same hull wherever it landed. Given enough time, anyone can become a good shooter. I’m 70 years-old and just this last summer, I again hit a coke can at 90 yards with the the .454 Casull on the first shot right after I shot the coke can next to it with my Ruger New Model Super Blackhawk in .44 Magnum on the first shot and again with witnesses.

    2. avatar Ray says:

      Hello S. McQueen,
      Your assertion “feels” as though it should be true but Jeremy S. already covered why practice is practice even when you are dry firing so I will cover the part about your assertion that the overall “thump” of the 10mm is a necessary and important component of shooting a 10 mm.

      “The pistol may be tthe same, but the entire experience is significantly different in 40 vs real 10mm in recoil, follow up, noise, muzzle-flash, adrenaline flow, etc., etc.).”

      I have increased my recoil spring on my 10 mm setup (can’t remember if it’s 22# or 24#) and I can’t really detect a difference between shooting a 10 mm Buffalo Bore G21 conversion and my G21 .45 ACP.

      Maybe because I started shooting with a .41 Magnum (Ruger Blackhawk) and I’m used to dealing with a little bit of a kick, but a friend of mine agreed that it was surprising how manageable my 10 mm setup was to shoot.

      So I disagree with your assertion that you have to deal with the “thump” of a 10 mm to shoot a 10 mm.

  39. avatar dirtfootdan says:

    So the .40 doesn’t sit any deeper in my glock 20 than the 10mm. It’s not a Gen 4,.it’s a glock 20.
    The extractor does not pert rude forward whatsoever because the brass still sits up against it, that does not change for either brass, meaning the headspace remains the same; otherwise the firing pin wouldn’t be able to reach the primer.
    I’m glad it’s working for you and that you are this confident in doing so.
    Personally I’m going to buy myself a .40 cal barrel for under $100.- to keep any mishaps from happening.
    Besides it doesn’t do me any good to practice for a 10mm burst with a 40 cal,.. it’s not identical, although very similar.
    In case of emergency with no 10mm ammo on hand this is definitely a good thing to know,…
    Keep on keepin on! Pew pew!!

    1. avatar Jeremy S. says:

      It does sit deeper 99% of the time. In fact, .40 in a 10mm sits deeper in the chamber than .40 in a .40. This is precisely because it isn’t headspacing off of the case mouth, which obviously stops the round from going deeper into the chamber, but is being held in place by the extractor. This means the action is closing as far as it can physically close. The most obvious way to “prove” that this does, indeed, mean the round is deeper in the chamber is to fire a few rounds of .40 through a .40 cal Glock (e.g. G22) then fire a few rounds of .40 through a 10mm Glock (e.g. totally unmodified G20). You’ll notice that the case bulge — that “Glock smile” — is clearly shallower and smaller in the 10mm barrel. It isn’t a chamber support thing or a difference in the barrel, it’s the fact that the round goes deeper into the chamber. Firing 10mm in a 10mm barrel — even a .40 S&W power level load! — also produces a much larger “Glock smile” than firing .40, and this again is due to the case mouth bottoming out in the chamber and extending the rear of the cartridge out the back of the chamber a little bit farther.

  40. avatar nmiller says:

    So I can probably run 10mm in my 40 then, right? Sorry, couldn’t resist…

  41. avatar Chris Turner says:

    Quick Question…
    Would it be worth, putting a G20 Barrel in a G22 or G23???
    For the above mentioned reasons obviously.

  42. avatar kain says:

    I have actually shot .40 through my glock 29 and no issues(all stock)

  43. avatar Bhernandez says:

    Enjoyed the article. I own a Glock 20SF. shoot full power loads thru a Wolf barrel. there is a drastic difference with the case head support between the glock and the wolf. with hot loads in the glock bbl, you get the smile, in the wolf, total head support, no smile. I can put the fired empty case (shot from the Wolf bbl) into the glock bbl chamber, and it rattles around.

    I have shot many a round of .40 SW thru my 20SF with zero issues. there are 10mm revolvers that utilize moon clips to load and fire, using a .40 in a 10mm place is like you said, a 38SPL in a .357 chamber.

    recently, i tuned up my Glock with a set of Wolff springs to control the recoil of my full power 10mm loads. it makes the load velocities more consistent with smaller standard deviation. but…my .40SW no longer cycle the action, the recoil spring is just to heavy.

    so next time i go to the range, i will carry the factory 10mm spring, which cycle the .40SW just fine.

    On your point 1-5, i concur will all but number 1. i know my 10mm don’t really headspace on the case rim, and the .40SW are held to the same position. you can pull the slide and barrel and insert an empty .40SW or 10mm case to see for yourself.

  44. avatar John Furman says:

    .40 S&W does NOT work reliably in 10mm Glocks. I’ve tested this myself with both factory and aftermarket Glock barrels in a Glock 20, 29, and 40 MOS. Here is what happens when you shoot .40 S&W from a Glock 40 MOS. https://youtu.be/O7wg5ppy4yk

    1. avatar Jeremy S. says:

      It’s possible that the G40 doesn’t run well due to the heavier slide (plus weight of an optic), but I doubt it. The video you linked said nothing of caliber being used, and the implication is it’s jamming on 10mm. But you’d be the only person in many dozens I’ve ever heard claim it isn’t completely 100% reliable in a G20/29 and I don’t mean to be a total jackass but I honestly doubt you’ve tried it. Nothing personal, but I’ve seen many thousands of rounds of .40 shot through a bunch of factory 20s and a couple 29s (including my own) and have not witnessed one single stoppage, period. Your typical .40 is within 10% of the power level of your typical 10mm and way, way within a Glock’s ability to run reliably with the factory spring.

  45. avatar John Z says:

    Well this just saved me some money. I was about to buy an extended length 10mm-40 S&W conversion barrel from lone wolf for my Glock 20 and have them thread it for me as I want to use a good 40 subsonic round with my Octane 45 for home defense but still be able to swap back to 10mm for woods walks. Now I just have to buy a 10mm threaded barrel from them for $100 and I can shot either one. Thanks

    1. avatar BbHrdZ says:

      This is OK for the range, but don’t consider a .40 out of your Glock 20/10mm for home defense.

  46. avatar Steve H says:

    Anyone try 40 cal from a MechTech 10mm CCU for the G20?

    1. avatar Jeremy S. says:

      I did. It’s mentioned in the MT review 😛 http://www.thetruthaboutguns.com/2014/12/jeremy-s/mechtech-c-c-u/

      1. avatar Steve H says:

        Exactly what I was looking for, just had to ask!
        Many thanks!

  47. avatar Eric says:

    Has anyone tried this same .40 cal ammo in a EAA Witness Hunter 10mm gun? And we are all OK with the extractor being playing a big role in keeping the bullet in place?

  48. avatar JP says:

    I agree with the glock 10mm having a thicker slide and barrel giving the glock 10mm much more strength and recoil control over the glock 40.
    I shoot 40 in my g29 gen 4 with a kkm 40 conversion barrel and it works flawlessly and with very soft recoil. I think you could be correct in saying many factory rounds might be a hair short and held in place by the extractor. My only point of disagreement is how can a .40 round be deeper in the barrel of a 10mm when the extractor is holding the round against the breach face of the slide.

    1. avatar Jeremy S. says:

      “My only point of disagreement is how can a .40 round be deeper in the barrel of a 10mm when the extractor is holding the round against the breach face of the slide.”

      The limitation of a 10mm inserting deeper into the barrel isn’t the breech face, it’s the end of the chamber. A 10mm round bottoms out on the end of the chamber via the case mouth. It cannot go deeper into the barrel than that hard stop. It’s going to stick out the end of the chamber by however much its length dictates regardless of what the breech face wants to do. Let’s say the case is way too long…well, the gun may not lock up into battery and a bunch of case will be hanging out the back of the chamber. Let’s say the case is just a little too long…the gun may go into battery enough to fire but not actually be completely closed up. An average 10mm round may space the breech face slightly back from where it would be if it closed on an empty chamber instead, and any amount it’s pushing the action back from its most-closed-possible state is directly related to additional case sticking out the chamber. On the flip side, nothing stops the .40 S&W in a 10mm chamber, as the chamber is too long for the case. The action is always going to be absolutely as fully closed as it can possibly be. So, the round will be as far into the chamber as the action will allow it. That may happen some or most of (or nearly all of) the time with 10mm as well (as you mentioned, there are even cases where a 10mm round isn’t going to headspace on the mouth but will just float off the extractor anyway), but it certainly will also happen more often than not that a 10mm case comes to a hard stop at the end of the chamber and sticks out the back enough that the action doesn’t close as fully as it would on a .40. Meaning the 10mm didn’t go as deep into the barrel as the .40 always will.

  49. avatar Kevin says:

    I shot 40S&W out of G20, reasonably accurate. But there is the brass to face problem when shooting 40SW. Almost every single brass fly straight to the face…Is there a way to fix this for 40SW?

    I don’t have such issue when shooting 10mm, G20 eject 10mm brass with authority to about 10 ft away.

  50. avatar Jeff Stanley says:

    Great article, thanks. I’m sold on getting a G20 and a G29 now. And a MechTech carbine upper too (although we’ll see about the forty through that)……

  51. avatar Donald Qualls says:

    I just bought a new EAA Witness in 10 mm (it’ll be here in a couple days). I bought a box of Sig Sauer 10 mm rounds — 185 grain, listed at 1265 ft/s on the box. I also bought a bulk pack of 200 rounds of .40, 185 grain at 1050. I did that because the .40 was not much more than half the price of the 10 mm, and I’d read this article months ago and knew I’d be okay. Oh, right, it’s not a GLOCK. But, if it doesn’t have .142″ of firing pin protrusion, it’ll be safer as well as cheaper with .40, for the same reasons a 10 mm GLOCK is safer. I saw an article saying a S&W would fire the rounds that slipped past the extractor, and showing that they had about 3 or 4 in 100 do that — but as long as the firing pin won’t reach a slipped round, that won’t matter, and if I have to, I’ll consider shortening the firing pin a few thousandths so it won’t. Meanwhile, I’m looking forward to keeping 10 mm in it for home defense and for pigs or coyotes when I’m out hunting deer, but practicing with affordable .40 most of the time.

  52. avatar Harold says:

    Wow gun folks are a bunch of closed-minded folks for sure.

  53. avatar Harold says:

    How is 10mm from 4″ to 6″ barrel bear defense? I mean how many 220 grain projectiles do you have to properly place in a 300 to 500 pound attacking bear to get him/her to stop chewing on you?

    1. avatar BHernandez says:

      the answer is: as many as needed. my G20SF is a 6inch wolf barrel, i shoot 135gr HP in a starline case. the velocity is 1630fps with muzzle energy of just under 800ft-lbs. with 15+1 rounds, this is my field carry.

  54. avatar Dariusz Nowak says:

    Any comments on firing 40S&W through Glock 40 (10mm Auto) – is it going to be similar to Glock 20 ?

  55. avatar Papadopoulos says:

    Would you ever try turn your Glock 20 to full auto with a kit and the try to empty a 0.40 magazine with it?
    What is your opinion?

    1. avatar Alan C says:

      Yes, yes and Yes. I’ll call it the 20/18, hopefully ready by next year ; )

  56. avatar Alan C says:

    Thank you for this excellent article! I like Glocks for only two reasons. Magazine capacity, and modular ammo diversity. I will be getting a G21T slide and 460Rowland barrel to add another level of flexibility to the already excellent 40/10mm lineup. After three years of dealing with Glock smile on Starline 10mm cases, I will be thrilled to start cranking out hundreds of 180gr fmj 40S&W rounds on my AP press. I’ve fired about 300 rds of 40 through it to verify this and no stoppages, no FTE’s, no kaBooms, acceptable accuracy at 50m. Great writing and just one more reason to love my G20. Now I just need to build a grip plug/magwell shim to use the 31rd G22 mags. Thanks again!

  57. avatar Jim says:

    Thanks for sharing your experience. You’ve convinced me to order a new Glock 40, based on the ability to fire both 10mm and .40 S&W. Weather I’ll do it with or without a barrel conversion remains to be seen. But at least now I know my options.

    Thanks again

  58. avatar Jugchoke says:

    As I mentioned above, I don’t believe that the cartridge is being held completely tight and flat by the extractor.
    There is still movement available and likely just a bit of tipping to be straightened out yet.

    That is, UNTIL, that firing pin hits the primer. Then it is driven forward just enough to take all the “slack” out of the stack height, and start the firing sequence.

    I have seen this on shotguns with too much head space. (But not enough to blow anything up.)
    And when this happens, felt recoil increases fairly drastically because of that plastic shell not gripping the walls and the whole thing slamming back against the bolt face. (Which leaves a deep firing pin indentation, BTW)

    The difference here is that as the thin brass pistol hull expands, the walls of the cartridge lock onto the walls of the chamber, (with the cartridge driven completely forward).
    So, I do believe that the .40 is sitting deeper into the chamber when it actually fires, and thus has some bit more support just as Jeremy says.

    And if the pressure is still great enough to drive the head back flat against the breech face, there has already been a significant lowering of the pressure. Thus, NO, or at least, less SMILEY FACE bulge!

    One way to check, would be quite precise measurements, looking for any case lengthening.
    Precise, because it would likely take numerous firings and reloadings to show any sign of incipient head separation tendencies.

    I have no idea how many reloads any of you are getting out of the brass, only the .40 SW of course, but I would expect it would take at least five and maybe even ten to show up.

    Think about it, if the actual 10mm doesn’t quite bottom out on the case mouth, the same thing is happening.
    Not yet shooting a 10mm, (but that will change later this week), but I have seen the same problem on 9mm non fully supported chambers. Never checked the various case lengths, but have been using just as many different brands as I can find for checking out ammo compatibility.

    I just haven’t checked it out against the different brands, or actually done the measurements.
    If I considered it important in this 9mm case, I would have another project on my hands.

    As it is, I will be receiving my first 10mm later this week, Glock 40 MOS, and I damn sure will be running a project there, after having read this. I figured that at 81, I needed a new caliber to play with!

    1. avatar Jeremy S. says:

      I think it is held tight and flat. The Glock extractor is under significant spring tension when it’s on the rim of a cartridge and it’s squeezing it quite hard against the breech face. Even without the chamber keeping the round perfectly aligned, it stays dead straight and flush on the breech face: http://cdn0.thetruthaboutguns.com/wp-content/uploads/2015/06/20150626_143520.jpg

  59. avatar Joan R. USMC Mjr, (Ret.) says:

    I am a Reserve Officer and a Retired Officer in the USMC. In North Carolina and in Fallujah, this combination of the 40s and the 10mms in my Glock 20G4 and 29SF, has saved my COs life and me when confronting someone on PCP. After my fellow officers were hitting this suspect with 9mms that were either passing right through him or would graze him, I was able to get on scene just as he was heading for a public school. 2 10mms in the head and 2 40s in the chest, dead before the first 40 hit his chest in rapid fire. You also have to consider the target’s CONDITION, if they are on some type of drug that will crank up their adrenaline making them near impossible to take down unless you hit them with a great bullet placement. I’ve used this combination during Desert Storm and its not failed me in anyway. Thank You for this article, This backs up also what my DI used to tell us during range practice, ” Its not how big the bullet is, soldier. Its where you place that piece of lead in the target so they can’t get back up!” He taught me this and continued to teach me throughout my time in the Marines as a female officer. We lost him last night and I’m planning on heading in full dress to pay my respects to this man who has shown me how to save myself and those who are around me. I will always use this combination.

  60. avatar AverageJoe says:

    In one range trip, I shot 500 rounds of factory 40 sw ammo through a glock 29 and a glock 40, with about 1/2 through each gun. Both the G29 and the G40 ended up with chips knocked out of the inside of the ends of their barrels. Both guns were fairly new, and were cleaned and lubed prior to the range trip.

    I would not recommend shooting 40 sw through unmodified glock 10 mm pistols, and I would only do it again in an emergency situation.

  61. avatar Steve Kim says:

    THANK YOU for a thouroghly written explanation about shooting 40sw out of a 10mm.

    I recently got my hands on a Springfield Omega 10mm (6 inch ported version)
    I am wondering if I can do the same?

  62. avatar gigo gio says:

    relying on the extractor for headspace is retarded..

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