Abstract: The firing pin (17) of a semi-automatic arm is so adapted and arranged in a firing pin chamber (15) that, in addition to being movable axially between a neutral position and a striking position under the action of a hammer (34), said firing pin is movable between an inactive position in which the striking end (25) is staggered radially with respect to the percussion cap of a cartridge (15) inserted in the firing chamber, and an active position in which the striking end is positioned axially in line with the percussion cap. This movement is produced by a trigger actuated trigger bar (35, 36) which releases the hammer after having actuated the firing pin. Since firing can take place only when the firing pin has been caused to assume the active position under the action of the trigger, the arm has a very high degree of safety against unintentional firing.