Autoloaders and semi-automatic shotguns take the pump-action idea one step further, using similar mechanisms to those employed by machine guns. As the designs get more complex and have more moving parts, the chances for operator error, misfire and jamming increase dramatically. Autoloaders are considered less reliable than pump-action and break-action guns.
Recoil-operated autoloaders use the force naturally generated by recoil from the firing process to eject the spent cartridge, get a new one from the magazine and ready it in the chamber. In this case, the explosion from the cartridge forces both the barrel and the bolt to travel a couple of inches backwards. This ejects the spent cartridge. The barrel and bolt hit springs that send them forward again, and the bolt strips a new cartridge into place on the way. The barrel and bolt lock back into place and are ready to fire again. There are also short-recoil systems that work similarly but with a greater separation between the movement of the barrel and the movement of the bolt.
The gas-powered variety of autoloaders work like assault weapons. For a detailed explanation of these systems, see How Machine Guns Work: Gas System.