A firearm apparatus includes a gun housing which includes a barrel support channel between a front gun housing portion and an intermediate gun housing portion. A reciprocating barrel is housed inside the gun housing and includes a piston-sleeve-contained barrel portion slidingly received in a piston sleeve which is housed in the front gun housing portion and in front of the barrel support channel. Proximal portions of a barrel piston head, the piston-sleeve-contained barrel portion, and the piston sleeve form a pressure space into which a portion of high pressure combustion gasses from a fired cartridge enter. The high pressure gasses propel the barrel forward until the barrel piston head reaches the end of the piston sleeve. Then, a recoil spring returns the barrel to its original position. As the barrel moves back and forth, a fresh cartridge is automatically loaded from a magazine into a barrel chamber portion, the cartridge is fired, and the spent cartridge is automatically discharged.