Gyrojet weapons fire like regular firearms, but the bullet is also a small rocket, which gives the weapon improved performance. The downside is inaccuracy and lack of amunition capacity.
Somewhat unusual, gyrojets fall inbetween the power of energy weapons and the low price of firearms. Ammunition for gyrojet weapons is quite expensive, though the weapons themselves are cheap. Still, they are popular in certain cultures, where proper energy weapons are outlawed. Middle-class home-defence use is the most common niche - the gun price is affordable, and the ammo cost is largely irrelevant. Of course, such a homeovner is unlikely to be able to use his gyrojet well.