firearm axis the direction and amplitude of rack 20 motion are determined by guide rails 21 and 22 which are parallel to the firearm axis and are fixed on frame 15 the ends 23 and 24 of rack 20 fig8 and 9 come to rest alternately on said guide rails 21 and 22 in order to maintain the switching devices 16 in a fixed and determined position during the motions of slide 4 rack 20 is locked on either one of guide rails 21 and 22 by protruding slugs 25 fig1 and 11 which retract when pushed in by shoulders 26 cut in studs 27 that are secured on frame 15 and whose side faces act as stops combined with slugs 25 so as to intermittently freeze the translation of rack 20 when a cartridge 57 is fired slide 4 is slammed by piston 7 in the direction x fig1 and 6 and drags rollers 6 whose interaction with groove 5 and switching devices 16 up to the aft neutral position of said slide 4 causes cylinder 1 to move one fourteenth of a revolution owing to the kinetic energy built up in cylinder 1 and to the action of the recoil springs 10 slide 4 is propelled.