Most semi-automatic shotguns operate with one of two systems: recoil or gas. The recoil system is the most basic and works on the principle of Newton’s Third Law: “For every action there is an equal and opposite reaction.” After the trigger is pulled, the force pushes the breech bolt back, ejecting the empty shotshell and loading a new shell from the magazine. The bolt then springs forward and locks into place.
The gas-powered autoloader works in a similar way to the recoil system, but it incorporates a piston into the process, which is activated when the trigger is pulled. The gases expand behind the traveling shot and are captured in a cylinder underneath the barrel. Pressure from the gas then propels the piston backward, which drives the breech bolt as well. The spent shell is ejected and a new one is put into place.