The .22 Long Rifle (.22LR) is the most common cartridge in the world. Used for over a century to train people in marksmanship worldwide, the cartridge retains many features from the 1800s, most notably it’s large, protruding rim, which also contains its priming compound. This rim limits its capacity in most designs, as most are single-stacked at a relatively steep feed angle to eliminate “rim lock,” the phenomenon where the rim of a round catches on the rim of a round under it, preventing feeding and generally jamming up a gun. As a result, most .22LR handguns have a lower capacity than similarly sized 9 millimeters, despite the round itself being smaller.

But Kel-Tec came out with CP-33, it bucked convention by making a .22LR pistol that utilizes a casket magazine design to get far greater capacity than most of its competitors, thirty-three rounds flush and fifty with an extension. But how did Kel-Tec engineer such a design?

Some may credit some of Kel-Tec’s CP-33 magazine design to their earlier design work on the PMR-30 magazine, which also features high capacity with a .22 rimmed cartridge. But the designs are actually fairly different. Due to the larger nature of the .22 Magnum cartridge in the PMR-30, stacking them in four “columns” is rather infeasible, so the PMR-30 utilizes a double-stack design. The rims are kept separate and prevented from rim locking with cartridges on the opposite side of the stack via a plastic lip that runs down the center of the magazine, while the noses of the cartridges are kept in two separate channels to ensure proper feed geometry. At the top of the magazine, the cartridges are presented in a double-feed manner, feeding from both the left and right stack alternatively.

The CP-33 magazine, on the other hand, stores the cartridges in two double-stacks with a full length “partition” down the center of the magazine in a standard casket magazine layout. Geometry inside the magazine is used to prevent rim lock, and at the top of the magazine, both stacks are merged into an almost single-feed setup at the very top.

As it turns out, it’s possible to make a double-feed .22LR setup, if careful attention is paid into setting a correct angle for the rims and via nesting the noses together properly. However, double-stack .22LRs cannot be as space-efficient as double stacks with modern cartridges with rebated rims where cartridges are stacked with almost no space in between them, significant air gaps must remain in a .22LR stack due the rims. Kel-Tec is not the first to do this, magazines used in .22 LR pistol conversions by Jonathan Arthur Ciener were known to use .22LR double stacks to gain additional capacity.

Ciener magazines fit fifteen rounds into a full-length pistol grip, so it’s likely that the CP-33 is using a very similar round stack to the Ciener, fifteen plus fifteen, with three additional rounds where the stacks merge adds up to thirty-three rounds in a standard CP-33 magazine.

But how is the design of the pistol itself? The CP-33 is also unlike most pistols in that it features an upper section that extends far behind the grip. This is because in many ways the internals resemble a rifle more, with an internal bolt rather than the slide. The design placing the recoil spring behind the bolt leads to the massive rear section of the pistol. While unconventional and bulky, it does result in the pistol having a ton of mass which makes it extremely stable in rapid-fire.

However, the most interesting and innovative part of the CP33 remains the magazine. For those looking for a .22LR with insane capacity, Kel-Tec has made the perfect pistol. Their experience creating complex shapes from polymer really shows in the magazine, which would likely be very to create using regular metal stamping techniques.

Charlie Gao studied political and computer science at Grinnell College and is a frequent commentator on defense and national-security issues.

Image: Creative Commons.