A high-power civilian weapon.
Gyrojet weapons fire miniature self-propelled rockets. A regular explosive charge sends the rocket out of the gun, then the rocket ignites and gives it further power. The rocket needs a guidiance system to stay on course.
Gyrojet weapons are accurate, have long range, and do great damage. They are also rather cheap. The drawback is that the ammunition is both complex, bulky and expensive. This makes these weapons impopular with people who shoot a lot, like the military, but relatively popular among those who mostly want their guns to impress. Gyrojet weapons are thus rather common among police, security forces, gangsters and for home defense.