A pneumatic fastener driving tool (10) of the hand held gun type having means for positively insuring against accidental firing when the fastener magazine (20) is empty or detached, or when the nosepiece (24) is not pressed against a workpiece. The means comprises a linkage train including an intermediate link (66) carried by the magazine (20) and movable by a first link (26) reciprocably associated with the nosepiece (24) to move a second link (48) associated with the trigger assembly (38) to render the trigger (38) operational for firing the tool (10). There are locking means (88,76) associated with the feeder (80) of the fasteners for locking the intermediate link (66) in an inoperative position when the magazine (20) is empty of fasteners.