In 1960 a couple of entrepreneurs in San Ramon, California, Robert Mainhardt and Art Biehi, formed a company called MB Associates and began experimenting with alternative weapon technology. They felt that present handgun 
technology had reached its development peak, and sought a revolutionary change.
The result of their efforts is the weapon shown here, which although looks at first glance very much like an orthodox self-loading pistol was, in fact, a rocket-launcher. Originally developed in a 13mm size, the rounds it launched contained their propellant inside a stubby cartridge that stayed with the projectile in flight.
Spin stabilization was provided by angling four tiny rocket ports in the rear of the projectile. The barrel simply provided the initial direction. This allowed the barrel to be very lightweight, and was in fact drilled full of holes in order to allow the rocket exhaust to escape. Due to the fact that the round continued to accelerate after it left the barrel, the Gyrojet had poor short-range power, but improved dramatically with range. After about 100 yards the motor burned out, at which point the pistol-sized round had about 50% more power than the common Colt .45.
Although the design was unique, it met with little success. The round's solid fuel propellant was susceptible to humidity which would cause occasional miss-fires, or obviously more dangerous delayed ignitions. Additionally, humidity could cause incomplete fuel burn which would reduce the projectile's power.
Another problem was poor accuracy. To eliminate any resistance to the launching projectile, there was no rifling in the barrel. Accuracy of the projectile solely depended on spin stabilization generated by its four angled rear exhaust ports. Manufacturing tolerances in these ports made repeatable accuracy mediocre, and an occasional clogged port would send the round completely off course.
Although unsubstantiated, shown here is a drawing of an extended fin stabilized projectile design that MBA supposedly developed in hopes of improving the projectile's accuracy, although it was never produced. In this design the fin's stem first acted as the firing pin, striking and igniting a cap in the head of the projectile. As the solid fuel burned, the exhaust would cause the fin to extend until it was stopped by the ported collar in the rear of the projectile.
Due to its inherent reliability and accuracy problems the Gyrojet was never a commercial or military success, and only a thousand were produced between 1960 and 1969. Although, it did obtain some fame when it was featured as a futuristic spy weapon in the James Bond film 'You Only Live Twice'.