How do automatic guns reload?

Automatic weapons typically use recoil or a gas blowback system to eject the spent cartridge casing and reload the next cartridge from the magazine.

Blowback systems use the gas from the powder ignition of the cartridge to eject the casing and reload the next shell. A portion of the ignited gas is redirected to force the action back far enough to reload. Most automatic weapons use this design.

Recoil systems are just what it implies - the recoil from the ignited cartridge forces the bolt back and reloads the next.