Back in the 60’s, engineers were trying to push every envelope they could. Defense contractors were not exempt. The process of manufacturing firearms was a labor-intensive manual process that required significant human intervention by trained, experienced people. That labor bottleneck is what inspired Eugene Stoner to develop the AR-10 (which was adapted into the AR-15). It could be manufactured quickly on automated machinery to precise tolerances without the need for hand fitting, and thus (in theory) all of the parts would interchangeable.
The MBA Gyrojet system of firearms was another idea that was run up the flagpole.
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The United States defense industry pioneered the development of solid rocket fuel to make land- and sea-launched ICBMs reliable and safe. Two enterprising men, Robert Mainhardt and Art Biehl, decided to use that solid rocket fuel to propel a bullet. They formed MB Associates (MBA) and designed a rocket projectile and guns to fire it.
Since the Gyrojets generated their own thrust, there was no pressure to be contained by the gun. The guns were just lightweight (zinc alloy) perforated frameworks designed to hold and aim the projectiles. There were pistol, carbine and assault rifles made, and other versions were proposed, like a LMG and an underwater gun. Engineering-wise the handgun was really more of a proof-of-concept than a viable weapon; the magazine held 6 rounds, and had to be loaded from the top of the gun.
While a neat idea on paper, the reality of rocket-propelled bullets was fraught with challenges. Reliability was not convincing, especially not in the damp jungles of Vietnam (never officially adopted, some units were sent for testing). Accuracy was a challenge; since the rockets lacked the mass and velocity to travel along a tight rifled barrel to impart a stabilizing spin, the projectiles had 4 angled venturi tubes on the back to spin it. Manufacturing flaws compounded the accuracy problems, making the system even less accurate. When you factor in the long delay between 1) pulling the trigger, 2) the projectile being pushed backwards onto the fixed firing pin, 3) the fuel igniting and burning to sufficient velocity to overcome the tension of the hammer spring, and 4) the projectile actually arriving at its target, bad accuracy isn’t surprising.
A heavy bullet does more damage, but a heavy bullet is slow to accelerate (a trade-off that all ammunition must balance). As a result, the projectiles left the barrel at a leisurely 100 feet-per-second, and did not develop useful velocity until 15-20 feet downrange. Not exactly a selling point for a personal weapon. Rumor had it that you could put your finger over the muzzle and prevent the round from exiting the gun.
Another complication: the small rocket projectile had limited propellant capacity, which is why the gyrojet projectiles ran out of fuel at approximately 60 feet after achieving maximum velocity of 1250 fps or so. As any large-for-its-mass object, it was more affected by wind resistance than a dense lead bullet and would slow down drastically after its fuel was spent. And forget about a hollow-point projectile; fuel capacity was the prime consideration of the ammunition design. The pistols and carbines were originally chambered for 13mm projectiles; after the Gun Control Act of 1968 (which banned weapons of greater than .50, among other things) the ammunition was retooled to be 12mm. The assault rifle design used 6mm rockets.
The concept never took off, despite being featured in a James Bond film (“You Only Live Twice“) and a Man From U.N.C.L.E. novel. The challenges were too great and too numerous to be overcome, and the benefits too meager (much like the gas-turbine powered automobile). You can find these guns at gun shows, they are usually expensive, and the ammunition can go for $100+ per round (and no guarantee it will work, either). Until Aguila ran off a batch of 5mm Remington Magnum Rimfire ammo last year, I thought those gun owners had the worst regret every time they pulled the trigger. Now we know better.
But, there are still some people trying to solve the problems with the Gyrojet concept. Who knows, you might see a new version of the Gyrojet at the SHOT Show in a few years…