a liquid propellant gun system has an annular differential piston journaled for telescopic movement with respect to an annular control valve and to the chamber of the firing bore ; uses the piston , the valve and the projectile as valve means for controlling the injection of liquid propellant into the combustion chamber of the gun system ; uses a displacement type control means to limit the rate of the loading of liquid propellant and to provide a positive evacuation of misfired liquid propellant from the combustion chambers ; and incorporates a vernier control circuit to give direct control over the performance of the gun .