The XM-19 was one of a series of Special Purpose Infantry Weapon (SPIW) prototype designs in the 1960s and 70s – a project which was attempting to increase the hit probability of infantry weapons. Several different approaches were tried to this end, including firing multiple cartridges in very fast succession, firing single cartridges with multiple bullets, and firing flechette darts (both individually and in clusters). This magazine is for the XM-19, which was a weapon dating to the late 1960s. It holds 50 rounds (all of the SPIW weapons needed large magazines) and was made for the XM-645 cartridge – more on that in a moment.
The magazine is interesting in particular because of its spring. What could conceivably make a magazine spring interesting, you ask? Well:
That’s the spring – well, the two springs. They are a pair of flat metal strips rolled up like ribbons affixed to the top of the magazine body and pushing against the follower. As ammunition is loaded in and the follower is pushed down, the springs unroll and flatten out, but are always trying to roll up, thus providing the motive force to push cartridges up. Pretty neat, eh?
Now, the XM645 cartridge itself was a bit longer than a 5.56mm NATO cartridge, and fired a 10.2 grain flechette as at remarkable 4800 feet per second (that’s 0.66 grams at 1460m/s). The flechettes were stabilized in the barrel by a fiberglass sabot at the front, which was removed by a “stripper” device at the muzzle. This sabot and stripper created one of the problems with the XM-19…traveling at 4800 fps, the sabots were basically vaporized at the muzzle. That fiberglass vapor would often then get into the firer’s eyes and lungs causing significant irritation (and probably some sort of horrible lung cancer). For this and other reasons, the XM-19 was eventually dropped.
One other interesting side note of the XM-645 cartridge and XM-19 weapon was that it was a piston-primer system, which is a different way of saying primer-activated – the same mechanism as the first Garand rifle prototypes. Since the XM-19 would naturally use its own proprietary ammunition, changing it to have a moving primer would not cause logistical problems. Here are a couple views of the cartridges:
Note that the far right cartridge shows the primer in its fully extended (fired) position. The primer moving backwards like that provided the energy to unlock and cycle the action of the weapon. In the far left cartridge cutaway, you can see how the primer is made specifically to have this amount of movement upon firing. Unlike primer-actuated guns using standard ammunition, this cartridge incorporates an intentional limit of travel for the primer.
Here are an assortment of additional photos of the magazine: