Abstract: A fastener driving gun has a firing pin, a breach block, and a cartridge magazine strip guide. A cocking lever is pivotally mounted to the guide and has one end portion facing the firing pin and another end portion facing the breech block. The end portions are on opposite sides of the pivot. One end portion is engageable with the firing pin, and the other end portion is engageable with the breech block. The end portions are so related to the firing pin and the breech block and the pivotal mounting that when the breech block advances in engagement with the cocking lever, the cocking lever retracts the firing pin. A sear release pin is so related to the trigger that positive force to move the pin in a firing or opposite direction is possible by manipulating the trigger. A barrel assembly made of two pieces threaded together includes a longitudinally extending top slot receiving a reset pin that assists in control of barrel position and attitude for various conditions of the gun.