How A Machine Gun Works
Machine guns and other automatic weapons are described as recoil-operated or gas-operated. In a recoil-operated gun the force of recoil when a shot is fired moves the barrel backward a short distance. This movement actuates mechanisms that eject the empty shell casing, load a new cartridge into the chamber, and cock the gun. In a gas-operated gun a part of the gas from discharge of the explosive is led off through a port (hole) in the barrel into a cylinder. There the gas drives back a piston that actuates the ejection, reloading, and cocking mechanisms.
In all machine guns, firing and reloading are continuous as long as the trigger is held back. When the gunner releases the trigger a catch engages the lock or other mechanism at the completion of reloading and cocking, leaving the gun in firing position.