Semi-Automatic
Also: Semi-Auto · Autoloader
An action that fires one round per trigger press and uses gas or recoil energy to cycle itself, chambering the next round automatically.
A semi-automatic firearm fires a single round each time the trigger is pressed and then harnesses part of the shot’s energy to eject the spent case, cock the mechanism, and feed a fresh round from the magazine. The shooter does not work the action by hand, but must release and press the trigger again for each subsequent shot, which distinguishes it from fully automatic fire.
Most precision semi-autos are gas operated, tapping a portion of the propellant gas to drive the carrier, which is why they are often called a gas gun; others run on recoil or blowback energy. They cycle faster than a bolt action and ease follow-up shots, but with added mechanical complexity in the action and, often, a small accuracy penalty.