In the comments on Nick’s post about Franklin Armory’s new gas-operated .17 WSM AR-15, I played “Debbie Downer” on the likelihood of seeing a high capacity, double-stack magazine for the rifle. Why? Well, it’s not easy to get a rimmed cartridge to reliably feed from a box magazine in a semi-automatic firearm in the first place, let alone from a double-stack mag. End up with one rim behind the next and you’re clearing a jam. “Crashbbear” responded, “tell that to Kel Tec’s PMR 30.” Good point. There are actually two unique features in the PMR-30 mags that prevent feeding issues. Let’s have a look…

First, let me just preempt comments pointing out that other magazine-fed semi-autos chambered for rimmed cartridges exist. They do, yes. They are comparatively rare vs rimless chamberings, mostly in .22 lr, and much rarer still in double-stacked config. Rotary or tubular mags don’t count (solves the feeding problem, but not the high capacity). The underlying point here is only that the rims make life difficult. Let’s not forget that the desire to feed rounds out of a box magazine is the reason rimless cartridges were invented in the first place, as we all learned in Beetle’s post about his Borchardt C93.

There are certainly plenty of ways to get around this feeding complication. Rotary and tubular magazines, as mentioned, are good options. Magazines that hold the rounds at a pronounced tip-up angle are also common. This tilts the top of the rim back and moves the bottom of the rim forwards, ensuring that the rim of the top round is always in front of the rim below it.

http://my.opera.com/derWandersmann/blog/2012/02/13/im-calling-this-post-for-antonietta-because-she-asked-it-ma

Unfortunately this doesn’t solve our quest for the extra capacity gained by double stacking. Thus, we get to Kel-Tec’s PMR-30 magazines.

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The first special feature is an interior divider wall that runs nearly, but not entirely, the length of the magazine. This wall separates the rims of the rounds on either side and prevents things from becoming a jumbled mess. A slot in the follower allows it to ride the wall on three sides, helping to keep it nicely aligned.

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Before the top of the magazine body, the divider wall ends. As the next round starts moving up above the wall and towards center, following the taper of the mag body towards the feed lip on its side, it is simultaneously pushed forwards by the rear of the feed lip thanks to a nice angle made between feed lip and mag body. This forwards motion happens just at the right time. Right as the rims are about to get squeezed together, that top round moves to front.

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And that’s really it. Keep the rounds from getting jumbled up in the magazine so the rims don’t start overlapping in crazy ways. Make sure that the on-deck round is always in front of the next one. Fire, and repeat. We all have our complaints about Kel-Tec product availability, but they came up with a simple solution to a long-standing problem here and it happens to work. I, for one, would sure love to see even more high capacity, double-stack, rimmed cartridge firearms in the future!

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…and my apologies for some of the photos. This was spur of the moment and they’re cell phone pics.

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64 Responses to A Double-Stack Magazine For Rimmed Cartridges?

    • One of the puzzlers to me is that Mosin Nagants have a 5 round magazine and use a separator in it to prevents the top cartridge from overlapping the other four, but the Brits were seemingly too proud to ever do something similar for their Lee-Enfields. It is really annoying to have to pick up stripper clips the right way.

      • Mosin’s interrupter is probably the single most prominent feature of the design. It was added there for a reason, too – to remind, the rifle was designed by combining best pieces of two competing designs – one Mosin’s, the other one Nagant’s. Nagant rifle did not have the interrupter, and testing had shown it to be very prone to FTF as a result of that. It was largely why Mosin’s design was taken as the basis for the rifle, and only select few features of Nagant’s rifle were incorporated into it (specifically, stripper clip and magazine spring design).

        It’s also why the rifle was never actually referred to as “Mosin-Nagant” in Russian, but simply as “Mosin”. The pervasiveness of the double name in English is a mystery to me.

  1. The Romanian PSL in 7.62×54 also double stacks, but only 10 rounds. The design of the feed lips make you load one round at a time in the mag so the top round has its rim in front of the one below it, and a very stiff spring keeps it that way.

    I think the biggest advantage of rimless double stack magazines is the ability to load them quickly through stripper clips or other “jam them in” means.

  2. Ah, that Ruger. The Mk II is one of the best-shooting pistols ever made. It’s no wonder that they’re going for 350-450 in good shape these days. Now if only the ammo to feed them still existed…

    • Best shooting, yes. But not the best design for takedown and cleaning. Especially when the pivot pin for the mainspring housing/bolt stop works it’s way slightly out and catches on the frame when you try to disassemble. I don’t think I’ve ever cussed at any firearm as much as at both my or my sister’s Mark II’s

      • Agreed- it’s meant to be cleaned on a well-lit bench, not a tailgate or in the field.
        Thankfully, you can get the job done by taking the grips off and hosing it down with WD-40.
        (ducks incoming fire for the next 20 mintues)

        • “WD-40: WD-40 was never meant to be a lubricant – it was designed as a moisture displacer. It’s far too light for any load protection, has incredibly poor corrosion resistance, contains zero boundary lubricants, and rapidly oxidizes to form a sickly yellow varnish (hint: this is not good for delicate internal lockwork.) There are those who will defend this stuff vehemently, but then again you can still find people who think smokeless powder is a passing fad. Just. Don’t.

          Here’s a good read on gun lubrication:

          http://www.grantcunningham.com/lubricants101.html

        • I think he was referring to it as more of a cleaner, I know that has it’s merits. Don’t rely on WD-40 for lubrication though

      • I concur. Every time I disassemble my MKIII I end up swearing like Nixon at the thing.

        Ruger says that you can separate the lower frame from the barrel and receiver, but my best efforts with a rubber mallet still say otherwise.

  3. The SVD has an almost doublestack mag where the bullets are kept in a chess pattern.

    Back on topic. Kel-Tec should make a shotgun using a magazine like the PMR-30. If they made something like a Vepr with 10 and 20 round double stack mags they would have a winner.

    Heck, they should make mags like that for the Saiga and the Vepr.

  4. I eagerly await the Coonan Arms double-stack .357 magnum 1911.

    (I’m only half kidding. The grip on the thing would probably be ridiculously humongous.)

  5. Kel-Tec has always had good design work. Sometimes the execution of the vision / QC has been a bit of a rattlesnake for them.

    I still think a lot of their products are really good ideas. God bless ’em for daring to be different.

    • Gimme a 16 inch one. .357 Magnum loads seem to be getting their peak velocity somewhere around 16-18 inch mark, according to BBTI tests.

        • .357 SIG is not the same thing at all. It’s made for pistol-sized barrels and it shows. .357 Mag actually scales very well with barrel length, to the point of getting into .30-30 territory in terms of muzzle velocity and energy out of 16″+ carbines; not so with SIG.

    • I’m in IL, so if Ruger wants to make me a 20+ round SR22, I’ll take it. And yours, if you wish. 😉

      Actually, since you can already change out the follower to get up to 13 rounds, perhaps they could give us a 25 round magazine for it?

        • I’m pretty sure it’s 13. It’s a 3rd party follower that you swap in the magazine. It’s shallower to allow the extra rounds. The downside is that you lose the lock open on the last round since the replacement does not have the pin.

          I haven’t used it, but have heard good things about it.

  6. I’d like to see a 30rd double stack 10/22 magazine and I’m sure everyone else would too! Something that looks a little more M1 carbine and a little less like a 40rd AK mag!

    • Aren’t the 25 round Ruger mags double stacks? I’ve got 5 and they’re dead on reliable. The only problem I’ve got is that my son eats through the ammo when we use them. You know how 10 year olds are with the “bang” factor.

      • No, the BX-25 is a single stack magazine. It’s thick because the magazine well in the rifle is designed for the rotary BX-1 mag, but the rounds go in a narrow channel down the middle and they are exactly single stacked.

        • And the mag curve is sufficient to account for stacking those rims? Interesting. I haven’t had to dismantle mine to clean, so I wasn’t aware which. Thanks.

  7. The ruger 10/22 steel lipped mags are also double stack and pretty damn reliable. The same principle as the butler creek and ramhorn ones people have made for years, but the factory ones are proof the trouble is more in quality than technology when it comes to rimfire mags.

  8. Can someone get on a double stack shotgun mag please? Does Kel-Tec have that patented? Open Class 3gun shotgun are challenging when there are more than 20 rds needed (ie., drum). Running several long regular mags is an option but not ideal.

  9. It looks to me like this is a design that could be easily implemented in some of the AR-style magwell .22LR rifles such as the M&P-15/22 and Sig 522. There’s plenty of additional width in the well and the magazines themselves.

    • Feed angle is probably a big player here. If you look at the PMR-30 magazine, it looks like a fairly low angle. OTOH, you average BDM AR-15 .22lr mag has a feed angle that’s really rather high.

  10. In my book semi-rimmed is functionally rimmed (if not worse then rimmed). Hence why I have always been suspect of gas operated firearms that use a semi-rimmed cartridge.

    Seriously, there needs to be a rimless cartridge that is smaller than a .380 (caliber wise) but larger (caliber wise) and more powerful than a .22 mag. I know some already exist (5.7, .32 NAA, etc.) but those are not that popular or plentiful. I would love to see one of those already existing cartridges, or a new one, become as common as the 9mm . . . or at least the .32 acp.

  11. Very interesting design from Kel Tec, yeh maybe I may even see one. But 10 out of 10 for simple design.
    My 303Enfields are staggered rimmed cartridges. How did the 30 round Bren gun magazine work? They are not prone to jam.
    Lets see more bolt guns with detachable 20 round boxmags

  12. Years ago, I approached Para-Ordnance with a suggestion that they develop a .22LR pistol/conversion unit that utilized a hi-cap (maybe even 50-round) magazine. They were, after all, the company that introduced the high capacity concept to the 1911 design. As we all know, nothing ever came of it.

  13. I remember a video where Nutnfancy went out with a dude from Kel-Tec and they had a lot of trouble getting the PMR to go into battery after firing a shot. There were many instances where it just didn’t lock forward into the firing position and they had to give it a short push into battery.

    I respect Kel-Tec’s drive to innovate but the PMR does not strike me as a reliable firearm and I’m pretty sure it is just something inherent to rimfire in a semi. It is disappointing because I know a lot of guys were thinking it could be a good pistol for the field or even carry, but it was so unreliable I don’t know if I would use it to shoot a varmint.

    I guess my question is if the mag could cause the slide not to return properly? If not the mag/feeding then what was causing that?

    • I can dump two 30 rounds mags back to back without any problems. The only issue I experience is the slide doesn’t always want to lock open after round 60. Gotta let her cool down a bit which isn’t a problem as it does take a while to reload the mags. Of course the real problem is finding reasonably priced 22 WMR to feed it.

  14. What I want to know is where the heck did you get hold of a PMR 30 for the pictures? Kel-Tec gives a whole new meaning to the term “ghost gun.” (Actually, I did get to handle one once. It sits a bit odd in the hand, but not unpleasant. It’s that triangular grip to accommodate the magazine…)

      • Yes, I do own one and I did the review here: http://www.thetruthaboutguns.com/2013/08/jeremy-s/gun-review-kel-tec-pmr-30/

        RE other comments above and below:
        They are NOT hard to find, just [very] hard to find at MSRP or less. There are tons on GunBroker right now with Buy-It-Now prices (for brand new guns) between $550 and $595. MSRP is $415. So… that’s about the state of the market these days. They were going for $1,000 a year ago when everything was nuts. Although the build quality doesn’t feel like a super nice gun because of the feel of the polymer and the way it’s put together, they’re a ton of fun and mine has been very reliable for me. Considering how many other polymer guns are in the $550 to $650 price range, I don’t think the asking price on the PMR on GunBroker is too much really.

    • I generally only go to one show a year anymore, and it’s not something I go out of my was to look for, but I can’t think of a gun show I’ve been to since the PMR was introduced where there wasn’t at least one on a table somewhere. Then again, it’s only been a recent development that the SUB-2000s have dried up. Did see quite a few KSGs last year, though. I guess Kel-Tec is only shipping to Michigan?

      • What were the prices on those PMR’s at your gun shows? That may explain their availability. It’s a cool gun, but no way is it worth the $600+ that most sellers are asking.

    • Plenty of PMRs on sale on GunBroker. Same as all other Kel-Tec “ghost guns”.

      Yes, you will probably pay more than MSRP there. But MSRP is not some God-given right price. It’s priced at what people are willing to pay for them.

    • In the M&P 15-22, the rim side of the cartridges is offset slightly but the bullet side is straight up-and-down single stacked. It prevents a really curved banana mag from being needed, but it is not a double stack mag. Maybe you could call it a partial stagger. What you see through the window makes that clear enough. It’s my understanding that the Sig 522 is single stack, although I haven’t played with it in person. I mean, just look at the 25-round magazine. It’s as long as a 30-round AR-15 magazine. You’re saying that when you double stack 25 rounds of .22 lr it takes up as much space as 30 rounds of .223? Not a chance. If it were double stack and the same length it would fit like 50 rounds. I mean, the PMR-30 fits 30 rounds of .22 wmr (which is larger diameter than .22 lr) in the length of a handgun magazine. Ruger BX-25’s that were mentioned above a few times are also really long. If they’re actual double stack mags I’m confused as to why they’re so much longer than the PMR-30 magazine when they should be quite a bit shorter (5 less rounds and a smaller diameter cartridge).

      Anyway I knew there was NO way to post this and not see half of the responses centered around picking examples that are double stacked magazines for rimmed cartridges in semi-autos haha. Although some bolt gun examples were mentioned also (and I own a Lee Enfield myself). If we were listing all of the semi-autos that used rimless cartridges, this comment thread would stretch to the tens of thousands. There is an obvious and demonstrable issue with double stacking rimmed cartridges and, although there are a handful of designs that we can all point out as exceptions, it certainly doesn’t change that fact. And the novel design of the PMR-30 mags is really cool, whether or not gun X or Y was double stacked in a rimmed cartridge back in the day. I’ve never had a feeding problem with my Lee Enfield either 😉

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