The Gyrojet was one of the more creative and one of the most futuristic firearms innovations of the last few decades - unfortunately it wasn't able to prove sustainable on the market.
The idea was to use burning rocket fuel to launch projectiles, instead of pressurized gas. The advantage was that without the huge pressure of standard cartridges, a rocket-firing gun could be made far lighter and cheaper, as it had no need to contain pressure. The rockets would accelerate down the barrel as their fuel burned (and the 4 rocket jets would be angled to put a spin on the projectile for accuracy), and the weapon would actually have the most kinetic energy at something like 20 yards downrange, when the fuel was expended.
A decent number of Gyrojet handguns were made and sold (mostly as curiosities), but intrinsic accuracy problems prevented them from ever being taken seriously as weapons. The company behind the guns (MB Associates) went out of business shortly, unable to fully exploit their full range of ideas. One of those ideas was a carbine variant of the gun. A few hundred were made in two different models, and we have the chance today to take a look at one of the Mark 1 Model B sporter-style carbines.