Gyrojet

The Gyrojet is a family of unique firearms developed in the 1960s named for the method of gyroscopically stabilizing its projectiles. Rather than inert bullets, Gyrojets fire small rockets called Microjets which have little recoil and do not require a heavy barrel to resist the pressure of the combustion gases. Velocity on leaving the tube was very low, but increased to around 1250 ft/s at 30 ft. The result is a very lightweight weapon.

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 MBAssociates, or MBA, in order to develop Biehl's armor-piercing rocket rounds. Originally developed in a .51 caliber, the cartridges were self-contained self-propelled rockets with calibers ranging from .49 and 6mm to 20mm.

A family of Gyrojet weapons was designed, including the pistol, the carbine and a rifle, as well as a proposed squad-level light machine gun and the Lancejet, however only the pistol and carbine were built. The space age-looking carbines and an assault rifle variant with a removable grip-inserted magazine were tested by the US Army, where they proved to have problems. One issue was that the vent ports allowed 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.

Versions of the Gyrojet that were tested were inaccurate, cumbersome, slow loading, and unreliable (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.

When spin stabilization was chosen for the Gyrojet, the compact electronics used in today's smart bullets were not available. Both the cost and size of inertial navigation equipment have decreased greatly since a Gyrojet launcher was reviewed in Guns and Ammo Magazine in 1965.

Design
The inherent difference between a conventional firearm and a rocket is that the projectile of a conventional firearm builds up to its maximum speed in the barrel of the firearm, then slows down over its trajectory; the rocket continues to accelerate as long as the fuel burns, then continues its flight like an un-powered bullet. A bullet has maximum kinetic energy at the muzzle; a rocket has maximum kinetic energy immediately after its fuel is expended. The burn time for a Gyrojet rocket has been reported as 1/10 of a second by a Bathroom Reader's Institute book and as 0.12 seconds by "The 'DeathWind' Project."

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 stabilization 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. Spin stabilization is limited in accuracy as a targeting technique by the accuracy with which one can point the launching tube and the accuracy with which the orientation of the projectile is constrained by the tube. The technique requires the shooter to have a line of sight to his target. For this reason the Gyrojet has been made obsolete by modern miniature inertial guidance equipment which suffers from none of these limitations. Advantages of the Gyrojet design over a hypothetical Inertially guided small arms rocket launcher are that, if ammunition for both were manufactured in sufficient quantity, the Gyrojet ammunition could be less expensive than ammunition incorporating an inertial measurement unit. Gyrojet ammunition is of a smaller caliber than is likely to be produced as a rocket with a current technology IMU, considering the size of highly miniaturized IMU components that were available in 2007 and are now so small that accelerometers and gyros are included in tablets and smartphones. The rocket leaves the barrel with low energy, and accelerates until the fuel is exhausted at about 60 ft, 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.

In 1965, the manufacturer of the pistol claimed 5-mil accuracy (about 17 MOA, or about 4.5 inches at 25 yards), worse than conventional pistols of the time. 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 M1911, the Gyrojet was considerably lighter at only 22 ounces (625 g) as the structure was mostly made of Zamac, a zinc 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. 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.

Tests in 2003 claimed 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 suggested that the (secret) manufacturing process was designed to achieve this effect. However, independent analysis of those testers' own published data shows that their conclusions were incorrectly calculated. The projectile's acceleration actually started out low and continually increased over the bullet's measured flight.

Gyrojet MkI
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.

Gyrojet MkII
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.

Gyrojet Assault Rifle
Assault rifle variant with M16 type ergonomics tested by the US Army. This variant had full auto capability and a removable grip inserted magazine. To increase ammo capacity, it's possible this platform was chambered in the 6mm calibre.

Gyrojet Carbine
Came with a rifle type stock, slight ergonomic pistol grip and scope.

Gyrojet Derringer
Image Derringer pistol with an upper barrel chambered for the Gyrojet round.

Gyrojet Flare Launcher
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 seven flares and had an effective altitude of over 1500 ft. Its rounded-nose projectile was designed to ricochet through trees and clear an over canopy of branches.

Gyrojet Lancejet
An underwater firearm variant of the Gyrojet called the Lancejet was considered for use by the United States military, but the inaccuracy of the weapon eventually removed it from consideration.

Gyrojet Pepperbox pistol
An experimental twelve barrel Gyrojet pepperbox type pistol was made that was used in Murderers' Row and was planned to be used, but was not in You Only Live Twice

Gyrojet Conversion gun
A prototype gun that converted from a pistol to a rifle. Had a folding wire stock and a screw on barrel. Chambered in the 13mm cartridge format.

Users
Although the Gyrojet was not officially adopted by any government, a few privately bought examples were used during the Vietnam War.