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