Gyrojet

The Gyrojet is 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 1,250 feet per second (380 m/s) at 30 feet (9.1 m). 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.