A firearm includes a housing having a detonation chamber and a projectile-discharge barrel. A displaceable piston is mounted at the discharge end of the barrel and is arranged to be acted upon and extended by combustion gases expelled from the barrel. Forward movement of the piston is transmitted to the housing in a manner tending to counteract recoil. The detonation chamber is arranged to receive a compression-responsive cartridge. A cartridge-detonating bolt is mounted within the bolt for reciprocal movement between cocked and discharge positions. A bolt-follower includes a locking device which is operable to temporarily lock the bolt against return movement to a cocked position while enabling the bolt to rotate. A cam-like configuration of the working end of the bolt compresses the cartridge in response to rotation of the bolt. The bolt-follower is coupled to the displaceable piston such that detonation-induced forward movement of the piston shifts the bolt-follower to an unlocking position. Actuating linkage is provided for transmitting bolt movement to the loading mechanisms of the firearm.