A semi-automatic rifle includes a frame (2); a barrel (3); a sliding guide (9) mounted in the frame (2) and forming one piece with breech bolts (10) which make it possible to control the recocking and the locking of the rifle (1); an automatic recock device (12) includes a piston (13) mounted in a gas cylinder (14) whose chamber (15) is connected to the bore (7) of the barrel (3) via a vent hole (16), whereby the piston (13) makes contact with the head of a sliding guide (17) which is connected to the sliding guide (9) and which is mounted in a sliding manner on a guiding rod (19) which forms one piece with the frame (2) and which is mainly parallel to the axis of the barrel (3). A floating mass (25) through which the guiding rod (19) passes is mounted in a sliding manner in the axial direction of the guiding rod (19) between two sides (27, 28) of a housing (24) in the head of the sliding guide (17) and a return spring (29) is mounted round the guiding rod (19) and situated between the frame (2) and the floating mass (25). The head of the sliding guide (17) is at least partially made of technical thermoplastic material.