Automatic or semi-automatic firearm

A firearm comprising:

• A frame, a bolt reciprocally mounted along the longitudinal axis of said frame, a barrel reciprocally mounted along the longitudinal axis of said frame between a first and second position, said bolt having a cartridge receiving means for receiving and inserting a cartridge in the chamber of said barrel and a cartridge extracting means for extracting said cartridge after being fired, firing means for firing said cartridge, a bolt drive spring disposed to drive said bolt forwardly, barrel biasing means to bias said barrel toward said first position, cartridge supply means for supplying a cartridge to said bolt, said bolt arranged to receive the cartridge on its forward stroke and afterwards insert the cartridge into the chamber of said barrel, said cartridge fired when in said chamber by said firing means, said bolt being biased into operative engagement with said barrel when said cartridge is fired, said bolt and said barrel in mechanically free engagement relative to each other, a gas system means for controlling the movement of said barrel relative to said frame, said gas system means including at least one propellant gas port in said barrel adjacent to the forward end of the chamber of said barrel, said gas system means further including an expandable compression chamber, said expandable cmpression chamber disposed between a portion of said barrel and a portion of said frame, said compression chamber in communication with said propellant gas port whereby when said cartridge is fired, propellant gas enters said compression chamber to act on said barrel portion to initiate rearward movement of said barrel relative to said frame, said bolt moving rearwardly under the force of said cartridge such that the initial relative motion between said bolt and said barrel is substantially zero whereby said bolt and said barrel remains in mechanically-free contact during at least a portion of the rearward movement.