Gyrojet

The Gyrojets were a family of unique firearms developed in the 1960s named for the method of gyroscopically stabilizing its projectiles. Firing small rockets rather than inert bullets, they had little recoil and didn't require a heavy barrel to resist the pressure of the combustion gases. Velocity on leaving the tube was very low, but increased to around 1250fps at 30ft. The result was a very lightweight weapon with excellent ballistics. Long out of production, today they are a coveted collector's item with prices for even the most common model ranging above $1,000. They are, however, rarely fired; ammunition, when available at all, can cost over $100 per round.

History
Robert Mainhardt and Art Biehl joined forces to form MB Associates, or MBA, in order to develop Biehl's armor-piercing rocket rounds. Originally developed in a 13 mm caliber, the cartridges were self-contained self-propelled rockets.

A family of Gyrojet weapons was designed, including a pistol, carbine, "rifle" and squad-level light machine gun. However only the pistol and carbine were built. The space age-looking carbine versions were tested by the Army, where they proved to have problems. One issue was that the vent ports allowed the humid air into fuel, where it made the combustion considerably less reliable. The ports themselves could also become fouled fairly easily, although it was suggested that this could be solved by sealing the magazines or ports.

Aside from a few Gyrojets tested by the United States Military, most Gyrojets were sold on the commercial market starting in the mid-1960s. These were Mark I Gyrojets, which launched a .51 caliber rocket, and ammunition was costly to produce and buy. In 1968 the U.S. Gun Control Act of 1968 created a new legal term, the destructive device. Under the new law, any weapon firing an explosive-filled projectile over a half-inch in diameter was considered a destructive device and required paying a tax and obtaining a license. The registration process was changed several years later, but in the interim, MBA created the legal Gyrojet Mark II, firing a .49 caliber rocket.

Versions of the Gyrojet that were tested suffered from poor accuracy, cumbersome and slow loading, and unreliability (at best, a 1% failure rate was suggested; users quote worse figures, with many rounds that misfired the first time but later fired). Possibly these disadvantages could have been overcome in time, but the technology did not offer enough advantages over conventional small arms to survive.

An inherent disadvantage was the very low power at short range.

Design
The inherent difference between a rocket and a conventional firearm is that the projectile builds up to its maximum speed in the barrel of the firearm, then slows down over its trajectory; while the rocket builds up speed from a standing start, leaving the barrel at a low speed and accelerating until its fuel is exhausted, then continuing its flight like an unpowered bullet. If the fuel burns at a constant rate&mdash;which need not be the case&mdash;the rocket accelerates at a constant rate. A bullet has maximum stopping power at point-blank range; a rocket has negligible energy at close range, but this increases greatly with distance (energy increases as the square of the velocity).

A firearm's rifled barrel must be manufactured to high precision and be capable of withstanding extremely high pressures; it is subject to significant wear in use. The Gyrojet rocket is fired through a simple straight, smooth-walled tube of no great strength.

Accuracy is increased by spinning the projectile. This is achieved for a bullet by being forced against spiral rifling grooves in the barrel. A rocket does not have enough initial energy to allow stabilisation this way. Spin stabilization of the Gyrojet was provided by angling the four tiny rocket ports rather than by forcing the projectile through a rifled barrel. Combustion gases released within the barrel were vented through vent holes in it.

The rocket leaves the barrel with low energy, and accelerates until the fuel is exhausted at about 60 feet (20 m), at which point the rocket has a velocity of about 1250 feet per second (FPS), slightly greater than Mach one, with about 50% more energy than the common .45 ACP round. While test figures vary greatly, testers report that there was a sonic crack from some rounds, but only a hissing sound from others, suggesting that the maximum velocity varied from slightly below to slightly above Mach 1.

Early tests showed the Gyrojet to be more accurate than conventional firearms, as expected due to the flatter trajectory. However in later tests accuracy was very poor; the difference seems to have been due to a manufacturing flaw in later production runs which partially blocked one of the exhaust ports, creating asymmetrical thrust that caused the projectile to corkscrew through the air.

About 1,000 of the "Rocketeer" model pistols were produced; a few saw service in the Vietnam War, and were featured in a James Bond book and movie You Only Live Twice, as well as one of The Man from U.N.C.L.E novels. At about the same general size as the Colt .45, the Gyrojet was considerably lighter at only 22 ounces (625 g) as the structure was mostly made of Zamac, a 7% silicon/aluminum alloy. The weapon was cocked by sliding forward a lever above the trigger to pull a round into the gun; the lever sprang back when the trigger was pulled. The lever hit the bullet on the nose, driving it into the firing pin. As the round left the chamber it pulled the lever forward again to recock it ready to fire again. The pistol lacked a removable magazine; rounds had to be pushed down from the open "bolt" and then held in place by quickly sliding a cover over them on the top of the gun. Reloading quickly was impossible.

It appears there is nothing inherently wrong with the concept, other than inadequate initial projectile velocity: a round can actually be stopped if a finger or even piece of cardboard is held directly at the end of the barrel, and will burn its fuel uselessly. This makes the Gyrojet totally ineffective at close range. The lack of speed often resulted in the rounds being unable to overcome the force of the reloading lever, resulting in the round failing to leave the gun.

Tests in 2003 found that the acceleration, rather than being constant, started at a high value and decreased, leading to velocities at close range which were not as low as expected, about 100fps at 1 foot instead of the calculated 20fps. The testers suggest that the (secret) manufacturing process was designed to achieve this effect.

At longer range velocity increases, so that the projectiles' trajectory does not drop as much as conventional ammunition, simplifying aim at longer ranges.

The Gyrojet principle was also examined for use in survival flare guns, and a similar idea was explored for a grenade launcher. The emergency survival flare version (A/P25S-5A) was used for many years as a standard USAF issue item in survival kits, vests and for  forward operations signaling, with flares available in white, green, blue, and red. Known as the gyrojet flare, the A/P25S-5A came with a bandolier of six flares and had an effective altitude of over 1500 feet(500 meters). Its rounded-nose projectile was designed to ricochet through trees and clear an over canopy of branches. It was quite effective.