Let’s step back to 1964 for a few minutes, into the midst of the SPIW development program. This was the US military’s attempt to increase hit probability from infantry rifles by, basically, changing focus from marksmanship to statistical probability. Studies had shown that hit rates in combat were pretty low, and the main driving theory was that you could get better results overall by using a weapon that fired multiple projectiles simultaneously. In a nutshell:
This theory found a number of different incarnations over many years of development, including 5.56 and 7.62mm cartridges with 2 or 3 stacked bullets per cartridge, flechette cartridges, multiple-flechette cartridges, multi-barrel guns, and super-fast burst firing (the H&K G11 is one descendent of these programs). Well, in the early 1960s this type of weapon was slated to replace the M14 – the M16 had been adopted just as a temporary place-holder until the SPIW (Special Purpose Infantry Weapon) was fully developed. The program requirements were pretty ambitious, though, and the companies working on it were having trouble meeting them. There are far too many different experimental guns that came out of the SPIW, SAVLO, and SAWS programs to cover in a single post (the best resource on the whole series of programs is Stevens’ and Ezell’s SPIW: The Deadliest Weapon That Never Was), and so today we’re just going to look at the 1964 bullpup design by Springfield Arsenal.
Actually, we’re mostly going to look at its magazine, because I recently happened to acquire one of them (a mag, not the whole rifle). The requirements at this point in the program called for a minimum of 60 round magazine capacity. The weapon’s lethality was to come from a very high rate of fire with a very light and very high velocity flechette projectile. The cartridge at the time was the XM144, which was a roughly 5.6x49mm case with a single 10.2 grain flechette projectile with a muzzle velocity of 4700 fps.
Winchester’s rifle could fire automatically (either unlimited fullauto or 3-round burst) at 1700 rounds/minute (as well as semiauto), so the 60-round magazine capacity requirement did make sense. But how does one carry 60 rounds in a magazine without it being really unwieldy? The XM144 was pretty close to the now-standard 5.56mm NATO in size, so a basic double-stack magazine holding 60 rounds would be both very long and have a spring too strong to effectively load by hand. Springfield’s solution was to make two 30-round magazines and fix them together, one in front of the other:
To prevent this rather bulky mag (6″ top to bottom and 4.75″ front to back) from getting in the shooter’s way to badly, Springfield designed their SPIW weapon as a bullpup, so the magazine would sit behind the shooter’s firing hand.
There is one other weapon I’m aware of that used this sort of tandem magazine design, and that is the British experimental Vesely SMG. This SPIW magazine is a more challenging design, though, because it uses a much longer cartridge.
The basic premise is that when both columns are full, a catch in the rifle holds the rear column down slightly, and the bolt picks up cartridges from the front column. As long as it feeds form the front column, everything works in a pretty typical way. Once that front column is empty (a 1.05 second burst), then a pair of tabs on the rear of the front follower trip the catch in the rifle, allowing the rear column of cartridges up into the path of the bolt. The rear column has lips to retain the cartridges, but they do not feed up out of the rear column. Instead, the bolt pushes a round from the rear column into the front column, so that as it round clears the lips of the rear column it is still being controlled by the front set of feed lips. A vertical divider in the front follower forces the cartridges to stay on the left or right side instead of moving into the center of the front column (where the round would pop free of the feed lips. Small follower scallops much like that of an M4 barrel aid in guiding the cartridges over the front follower. Once a round from the rear column is pushed fully into the front column, it then continues to feed like a normal magazine would, into the chamber and ready to fire.
This system seems to have worked at least reasonably well in a controlled environment, but it would have been a disaster in combat. That was acknowledged, and several more years were spend working to perfect the system. Finally, the ammo capacity requirement was dropped to 50 rounds, but most of the subsequent designs turned to using drum-type magazines instead of boxes. And ultimately, of course, none of the many experimental models were found to be good enough for service use, and we still have the M16 to this day.