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For some reason, I’ve got it in my head to convert my GLOCK 21 to shoot a wide variety of different calibers. Not that .45 ACP isn’t enough, but — well, it’s more a case of “because I can” than anything else, I guess. Last time, I fed this .45 ACP pistol some 9mm. Today I decided to plug the lightning rods into FrankenGLOCK’s neck bolts and resurrect it in a totally different caliber: 10mm . . .

10mm is a substantially more powerful cartridge than .45 ACP. While it’s not quite as big as the .45 ACP (10mm vs. 11.5mm, and about 180 grains vs. about 230) it packs a lot more energy into the cartridge. .45 ACP rounds typically deliver somewhere around 350 to 500 foot/pounds of kinetic energy; the 10mm typically starts at around 530 ft/lbs and climbs up to the 800 ft/lbs. neighborhood.

Converting a GLOCK 21 to 10mm is really rather easy; after all, the G21 is basically a .45 ACP variant of the GLOCK 20, a 10mm pistol. The frame is the same, and (as shown in the prior video) the GLOCK 21’s slide can accomodate barrels made for the G20. So … it’s almost a simple barrel swap to get the Glock 21 to 10mm.

There are a minimum of three things you have to change out to convert the G21 to 10mm: the magazine, the barrel, and the recoil spring. The magazine is the easiest part — just use a stock G20 magazine. Because the Glock 20 and 21 frames are the same, the mag fits perfectly. Note, this is also the same magazine you’d use for .40 S&W, and it’s the same one I used for 9mm.

For a barrel, I chose a Lone Wolf factory replacement barrel for the Glock 20, in stock length.

For the recoil spring, I chose to upgrade it. The slide on the GLOCK 20 is a little heavier than the slide on the 21, and with the 10mm being a more powerful cartridge, it can batter the gun if you don’t attempt to address that. Based on YouTuber MrGunsNGear’s G21->G20 conversion video, I chose a 20 lb. recoil spring from ISMI, and mounted it to a Lone Wolf stainless steel guide rod. The GLOCK 20 actually uses the same guide rod and recoil spring as the GLOCK 21, at 17 lbs., but the slightly heavier slide lets the G20 deal with the recoil a little better. With the lighter slide of the G21, a slightly stiffer recoil spring should help manage the 10mm’s stouter recoil.

I made no change to the extractor or ejector or any other parts. If you want the utmost in reliability, you may want to do so. I tested the pistol for my purposes (ammo testing) and found it to be satisfactorily reliable (i.e., I experienced zero malfunctions). If I was planning on making this a permanent change, I’d modify the extractor.

For testing purposes, I then loaded up the gun with CorBon’s 10mm DPX round, and shot a gel block with and without denim. The results were impressive. Even though this CorBon load is one of the lighter 10mm recipes, it still did fantastically well, with perfect expansion, excellent penetration, and a nicely savage wound cavity.

In the future I will be doing more 10mm tests on some of the more firebreathing rounds out there, but I can say that I was quite pleased with the results of this initial test. Recoil was increased, yes, but it wasn’t bad at all; certainly nothing like the monumental recoil of the S&W .460 XVR! The recoil was on par with a .357 magnum handgun — noticeable, and signficant, but definitely manageable.

Truth be told, I conducted the 10mm ammo testing on the same day that I conducted the .460 XVR testing. And while, yeah, the 10mm is a big boy gun, it was dwarfed by the .460 XVR. After setting off a dozen of those hand grenades, coming back to the 10mm felt like shooting a Red Ryder BB rifle in comparison. So maybe my perspective is a little skewed; maybe I’d have been more impressed by the recoil of the 10mm if I hadn’t dulled my senses on the megabeast revolver first. I’ll try to do a back-to-back recoil video between .45 ACP and 10mm next time I test some 10mm ammo.

I’m really happy with this upgrade. For minimal expense, I’ve added a new cartridge to my repertoire, and substantially upgraded the power of my full-size handgun (not only in terms of power per shot, but also in capacity, since the G20 magazine can accommodate 15 rounds of 10mm, where the Glock 21 can fit only 13 rounds of .45 ACP).

At this point, my GLOCK 21 can fire three of the major service calibers — 9mm, .45 ACP, and 10mm. And, with a simple barrel swap, I could add a few more (.40 S&W and .357 SIG, to name two). Not sure whether I’ll bother with them anytime soon though, as I’ve got my sights set on really ramping up the power game — .50 GI and .460 Rowland are on my wish list, but my next conversion will take this even further than I was planning by converting the GLOCK 21 into a 10mm carbine rifle. Stay tuned.

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    • I wonder if it is safe that the author used a G20 10mm replacement barrel in a G21?

      Lonewolf used to make “conversion barrel” for G21 to 10mm – and one dimension is certainly different on a conversion barrel.

        • EAA, is an importer, not a manufacturer. The “Witness”, is the brand name EAA sales them under. Tanfoglio makes them, and markets them everywhere, except in the U.S., under the umbrella name, “Force”. I have several Tanfoglio pistols, including an older TZ 75. Before EAA came into being, the two importers in the U.S., were Excam and F.I.E. Tanfoglio made the same pistol for both companies, Excam sold the TA 90, and F.I.E. sold the TZ 75. I have owned both, along with a first year, 1992, Tanfoglio/EAA FAB 92 steel frame. I currently own a 1984 TZ 75 9mm, a 1995 Tanfoglio/EAA Witness Stock .38 Super, a 1997 Tanfoglio/EAA Witness .45, a 2013 Tanfoglio/EAA Witness .45, and a 2015 Tanfoglio/EAA Witness Elite Match 10mm. With the exception of the TZ 75, which has the Tanfoglio name on it, the four Witness pistols, all have the EAA WITNESS name, and either Rockledge or Cocoa, FL, but they all have made by F.T. in Italy on the frames. F.T. is short for Fratelli Tanfoglio. Now, get your facts straight.

  1. Having the ability to quickly shift calibers in one handgun you shoot with regularity may be VERY advantageous when the SHTF and you have to make do with whatever ammo you have or can scrounge to survive.

    • That’s the best part about the Glock: they are like LEGOS – easy to swap out parts and easy to reconfigure.

      My original was the Glock 20, but I purchased a complete slide for the Glock 21, which was an easy swap. And the best part about going from the G20 to the G21 is that the 10mm magazine will also feed .45 caliber rounds easily and without any problems.

      There’s even more: Lone Wolf sells conversion barrels for (in my case) the G20, to accept .40, .357, and 9×25 Dillion.

      Although the Dillion is rare, and not likely to be a useful option if SHTF, it’s a fairly interesting round (9mm bullet on necked-down 10mm brass) with plenty of velocity and energy, both surpassing that of a 10.

      The good thing about a .40 is that it can also be loaded into a standard 10mm mag.

      These options are awesome – you can carry 5 calibers in one firearm. Swapping barrels or slides can be done in seconds!

  2. Great video, thank you. I love the Corbon DPX but it is just too darned expensive for me. I will stick with a cheaper alternative that I can afford to shoot a couple of hundred through my carry gun for reliability testing. The Hornady Critical Defense ammo is my choice.

    John Davies, Spokane WA USA

    • The 20’s slide is a little beefier, but not much. I weighed them both side by side on my postal scale, so … not exactly the right tool for the job, but it gave me a good idea. I don’t remember the exact weights but they were something like 17 oz for the G21, and 18 oz for the G20. I reweighed a few times to make sure that I was reading it right, and I got a consistent 1 oz increase for the G20’s slide. So yes it’s heavier, but only a little; I would think the 20lb recoil spring should perhaps be enough to make up for it.

  3. Try Underwood and Grizzly ammo, better specs than anything corbon can do. I about threw away the corbon powerball 357 sigs I had they were so worthless.
    Definitely get the extras to do .40 and 357sig, superior calibers to the 9mm and 45. Although nothing competes with the 10mm in semi I don’t shoot it anymore because for sd it’s overkill and dangerous in more than one way. For woods nothing beats a 460 or 500 IMO.

    Want to see a frankenglock show? Pick up the Lonewolf catalog and browse, most goofy mods ever. Reminds of a lowrider catalog with a bunch of crap one should never put on their car, or gun.

    • I shot a few types of Underwood out of it, but I only had hardcast solids in Underwood. I am now testing some DoubleTap rounds; just got back from firing some 125-grain copper bullets that clocked in at 1600 fps. That’s pretty potent!

      • I have seen quite a few reviews recently for all copper 10mm (not naming names) and keep wondering… is this a thing? I dont see as many all copper reviews for other calibers like 9mm. Are all copper for 10mm just newer so nobody reviews lead HP?

        Besides the extra $$$, dont these fragment at higher power? Is there any advantage of copper over lead? I keep wondering why i’d pick all copper over lead HP with the same weight and speed, or over the Hornady crit duty or crit defense.

        • All copper is definitely a thing. Copper bullets are typically more expensive than poured lead, but machined copper can do things you’d never get lead to do. Lehigh Defense makes some very interesting copper bullets like their Maximum Expansion and Controlled Chaos and Extreme Penetrator. And the Barnes solid-copper projectiles have been excellent performers in just about every test I’ve tried them in.

          As for fragmenting — I’ve shot copper bullets from the pocket .380 up to the .460 S&W, and I’ve never seen a copper bullet fragment. I’ve seen the petals rip off of them (as you’ll see in my test of the 10mm rifle) but nothing like what lead bullets do. I’ve had lead bullets be completely pulverized and totally fragmented, but I’ve never had a copper bullet do that.

          There are still many excellent bullets made in lead, certainly. Copper isn’t necessarily inherently “better”, but there are some things you can do with copper that you couldn’t do with lead.

  4. It’s definitely interesting to see the versatility of the Glock. The 10mm is a performer, but a .40 Smith is perfectly capable of pushing a 155 grain bullet to 1250 FPS from a full sized gun.

  5. I heart 10mm.

    Corbon DPX 155gr look (to me) to be at the low end of the energy range (500 fte). I would not expect much more punch or recoil than .40 sw with them. The target loads I use are around 500fpe, they indeed feel like a bb pistol on my full size 1911 frame. .40sw in a lighter pistol feels to have more recoil. Not sure what the G21 weighs.

    Now the hunting loads I have for SD … day-um. Underwood and Doubletap make some loads not for the limp wristed.

    Honestly though I dont think I’d go with all copper bullets. I start to worry they’d fragment once one gets to the day-um level loads (700 fpe).

    I really dont know why 10mm does not get more love. You can go low, some loads come in at 9mm power, or you can go high.

  6. Yawn – switching the slides around on a Glock is a little like playing with Legos, isn’t it? “Look Mommy, I made a robot!” “Yes dear, very nice.”

    Try doing something really interesting next time – get a barrel chambered in .224 BOZ, or custom gunsmith a single shot in .45-70 – do something unique.

  7. I love my Glock 20 and I love that you are testing 10mm now. I’d really like to see how the self-defense offerings from companies like Doubletap and Underwood perform. I like the idea of a Gold Dot sitting on top of a Doubletap factory load, but I’m concerned that the slug isn’t built to perform at those speeds and will over expand or fragment.

    I love Federal’s Trophy Bonded JSP load as a mixed country/city load, but it’s become unobtainium and I’d like to have another option.

    Anyway, I love your testing, STB410, and I’m excited to see you testing 10mm. I’m currently using a 10mm Mech Tech upper on a G21 frame. I haven’t shot it much but I like it so far.

    • Glad you like it! I have tests from Buffalo Bore and DoubleTap 10mm ammo coming soon (as in… someday…)

      Your concerns about the speed are warranted; I’ve been testing some rounds from the 10mm carbine and yes, some of them disintegrate, some fragment, etc. So the bullet technology definitely has to be matched to the speed that they’re expected to travel at.

      • That’s interesting. According to Ballistics By the Inch, there’s not that much speed advantage to the 16″ barrel in 10mm. I don’t have the chart handy, but IIRC, the speed curve flattens off pretty dramatically around 8-9″ of barrel length. Did you try those same loads out of your Lone Wolf-barreled G21?

        (This performance is contra .357 Mag, which–IIRC–is still gaining speed at like 18″ of barrel!)

        • the speed curve flattens off pretty dramatically around 8-9″ of barrel length.

          If you look at the tables carefully, this depends on bullet weight / load. A 135 gr 10mm goes from 1400 to 1640 fps from 5″ to 18″, while a 155gr goes from 1240 to 1300. The .357 110gr Cor-Bon flattens out, while the 125 gr does not. I think this is where case size and powder charge matters a lot . .357 is a bigger case (27 gr capacity, vs 24 for 10mm). You would probably have to reload either caliber to optimize for a carbine. BBI does not show a 110gr weight, but I bet you could do better than what they show for 135 gr.

        • When Jeremy S. Posts his review of the Mech Tech CCU 16″ 10mm carbine upper, you will see the speed differences between a G20 4.6″ barrel and the 16″ barrel. Some show more of a difference than others. IIRC, these DPX were about 250 fps faster from the longer barrel.

  8. Great video and article!!

    This is exactly what I’m planning to do (including the .460 Rowland and .50 GI). Due to some local gun laws restrictions, I’m limited in the number of semi-auto handguns/rifles I can legally own. So, being able to convert a firearms in several calibers is very important.

    I’m glad to see that what I was planning to do with the Glock 21 is definitely possible and actually looks to run great.

    Thanks again!

    • Some of it is Glock’s engineering and some of it is the popularity of the Glock platform. Beretta’s Nano and Pico have a polymer frame separate from the serialized subchasis receiver. It’d be pretty simple to have multiple types of frames (small for CCW, large for range fun, small hands, large hands, etc). But none exist, probably because there aren’t enough Nano owners out there to make a viable aftermarket ecosystem.

      • Yes, pretty much like Sig with the P320.
        I think this is pretty much where several manufacturers are heading to. Having a “modular” system, where the serial number is in the trigger mechanism. Everything else, from the frame, to the slide or the barrel, will be interchangeable to users choices.

        This is important for LEO and Military to adapt one gun to several people, but also in some state/countries where the number of semi-auto guns are limited in number. This is definitely my case, and I’m glad some manufacturer are proposing some official solutions, and some manufacture (such Glock) that are so popular that we can find 3rd parties solutions. Either way, in a world where everybody wants something tailored to his/her preferences, choices, and tastes, it’s great to have options 🙂

  9. I just can’t make myself do it. I really want a reliable double-stack 10mm… But like the bumper sticker says “God, save me from your followers!” I just can’t take the stigma of “Glock people.”

    Probably settle for an 80% 1911 in 10mm instead…

    • Kind of a necro-post, but if you want a reliable, double stack 10MM, check out the Tanfoglio Witness large frame handguns. They come in .45ACP, 10MM and .38Super, and you can swap slides/magazines to swap calibers. Even easier than the Glock swap.

      Hell, EAA even sells complete slides separately for just that purpose.

  10. Just curious what gen of G21 you are using to do this. A friend of mine, and myself, have been discussing the benefits of a “flexible firearm” such as this.

  11. Is it possible to go the other direction and convert a Glock 20 to .45 ACP? Seems more logical to use the 10 mm “beefier” slide on the 45 lower pressure than the other. Also read that the 20 can shoot 40 cal with just a barrel swap.

    • Hello
      How well does the 10 eject with the 45 ejector. The 10 and 45 ejectors are a little different. I went the other way. I bought a Glock 20 then added a Glock 21 slide a barrel. Only issue I have is the ejector on the 20 hits the 45 further in on the rim and they eject over my head instead of to the right. How well does your 21 eject the 10mm? Since the 21 ejector is out a little further it must hit the the 10mm shell close to the edge. I also purchased 2 Storm Lake barrels, 5.30″ both with two ports at the end. I also purchased a standard size Lone Wolf .40 and a complete 460 Rowland conversion from Rowland. It included the barrel, comp and a 24# recoil spring.
      Great multi caliber solution.

  12. Can anyone comment on the demonstrated reliability of .40 fired through a 10 mm conversion barrel on a G21? Having read the article on .40 through a G20, it seems like the lighter slide of a G21 would be a good match for the reduced charge of the smaller cartridge, but maybe the extractor issue plays in. Yes, I have read the reasons some people put forward for not shooting .40 in a 10mm barrel and I don’t care, I am asking only about demonstrated cycling reliability here.

    • I have read it’s fine to do. Same diameter. 40 cal is essentially 10mm short. Can’t recall if extraction was a problem, but I do think the spring weight was reduced from the standard 17 pounds. The distance that the bullet has to travel before hitting the rifling has been compared to revolvers to allay fears on that. I would feel safer shooting 40 cal out of the 10mm barrel in a Glock 21 than shooting 10mm out of it.

  13. No one mentioned the 45 Super!!? It’s just a 45acp case that is made with thicker brass! And the only thing that you have to change is the recoil spring!
    The loading data puts the Super at the same energy as a 41 magnum which is definitely higher than the 10 mm! I’m not sure whether there are any ammo company’s still making it but I think Corbon and maybe Buffalo might have been the ones. It’s the cheapest conversion of them all! And we all like to “get more BANG for the buck”!
    (I love using that expression)!!!

    • You may want to change the firing pin spring to a heavy duty one to get that f/p out of the way quicker. The Super has been known to send the slide back so fast the f/p will drag on the primer on the way back. I used 21-22 lb recoil spring when I was experimenting with the .45 Super. It’s a lot of bang on a 1911. It’s quite a bit of bang on the shooter too. I abandoned it as I didn’t find it worth it in either respect and std. .45 auto is powerful enough for my needs.

  14. Just curious how the same slide can deal with the rather significant difference in base diameter for the cases [10mm based on .30Rem parent brass, just like 6.8SPC; .45acp based on 6.5Swede parent…] That’s like a .050″ difference in base diameter: that’s a LOT! Just amazed that using the G21 slide can have any reliability at all…

    • Bruce B. , the 45cal. extractor will not be 100% reliable but you can still use it if you don’t mind a few stove pipes but it only takes a minute to swap out the extractor and put in a new $20 10mm extractor


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