Rockets of a sort to be more exact. A gyrojet round is somewhat like a modern rifle bullet in shape, meaning a fairly pointed conical tip that expands to fill the rifles barrel, but hollow inside and tends to be the much longer than a equivalent cailber rifle bullet. Instead of their being a brass casing full of powder mated to the bullet, which when ignited is wat propels the round out of the rifle; in a gyrojet round, the hollow inside is filled with the powder. When ignited, it basically acts like a rocket i,e, the propellant burns and the gyrojet round goes forward. The gurojet round goes undersome expansion as well, and is spin-stabilized by the guns barrel rifling, much like a bullet does now.
Unfortuanly, some simple physics is the reason why gyrojet weapons arent produced, mainly in relation to the maximum velocity and the speed at which terminal velocity is reached. Gyrojets simply arent as capable of penetration as an equivalent cailber bullet. as both maximum velocity and speed at reaching that velocity are significantly lower than a bullet. Aerodynamics(the hollow in the base of a gyrojet can produced some spectacular inaccuracy at range), comparable ROF, weight, and more than a few design challenges havent presented themselves in the past.
Basically dont hold your breath in ever seeing in operational one, at least on a handheld weapon scale. The principles have been understood for years, but no ones going to build an inefficient weapon when standard bullet-catridge rounds work much better AND are much simpler to build
