Since the beginning of the modern era, the question of how to arm the increasingly large body of non-infantry troops has become both more pressing and more difficult to solve. Initially, these men could be armed with pistols, or perhaps submachine guns firing pistol calibers, but as the Cold War entered its third decade, an additional complication was added to the problem: Body armor. Enemy troops – especially encircling paratroops – equipped with body armor could potentially make already lackluster pistols and submachine guns almost useless in a serious skirmish. Weapons were (and still are) needed that could defeat body armor while providing enough firepower to repel an assault, yet still be small and light enough to be carried by troops whose primary mission did not involve pulling a trigger.
The United States Air Force faced a similar problem in how to arm its aircrews. Pilots of downed jets were increasingly vulnerable as enemy forces mopped up the essentially unarmed jet drivers. A better weapon, capable of fully automatic fire and firing more potent ammunition than standard pistol rounds, was requested, and several designs submitted. One of these was the “Davis Gun”, designated by Colt the IMP-221, a pistol-rifle hybrid with a bullpup layout, firing the recently introduced commercial .221 Remington Fireball round. This gun didn’t go anywhere, but the basic concept was copied by Mack Gwinn, an enterprising firearms designer, who produced what he called the “Bushmaster Arm Pistol”. The Arm Pistol changed out the IMP’s .221 Fireball for the standard 5.56x45mm NATO, resulting in a weapon as compact as it was loud.
Hank Strange got the chance to shoot one of these rare firearms recently thanks to Safety Harbor Firearms and Walter Keller; the video is embedded below:
Hank’s video does a very good job illustrating why the Arm Pistol concept didn’t really take off. A weapon with this configuration can be described as a thruster (the muzzle) connected to a swing arm around a pivot (the shooter’s wrist), with a counterweight (the receiver and magazine) on the opposite end. This results in a system that wants to “trebuchet” about the shooter’s wrist, creating massive instability and dramatically increasing the time between shots as the weapon swings wildly about the shooter’s wrist.
Though the Arm Pistol/IMP concept was not successful, Gwinn did use the same basic mechanism in his also-obscure Bushmaster rifle.