A semi-automatic firearm automatically loads a round into the chamber after the weapon is fired, but still requires a trigger pull for each round that is fired. For example, if someone were to shoot ten rounds in a semi-automatic firearm, they would need to pull the trigger ten times (once for each round fired). Compare this to a fully automatic firearm, which will continue to fire as long as the trigger is held or until it runs out of ammunition. Pistols, rifles, and shotguns can all be semi-automatic.

Semi-automatic weapons are sometimes divided into two categories. Those that fire from an open bolt and those that fire from a closed bolt. When the trigger is pulled, the open bolt flies forward, picking up a cartridge from the magazine and ramming it into the chamber and the gun fires. The closed bolt system moves the bolt forward and picks up the cartridge as the last phases of the previous cycle, and when the trigger is pulled again only the firing pin moves. The closed bolt system is generally more accurate, since the center of gravity changes relatively little. The open bolt system is almost exlusively used in submachineguns, as it also lets the barrel cool more quickly.

There is some dispute over the correct use of the words automatic and semi-automatic. Gun enthusiasts sometimes argue that the word automatic is often incorrectly linked to fully automatic fire and that an automatic weapon is simply any weapon that chambers a new round during the extraction of the previous cartridge's casing. A semi-automatic would thus be a weapon using the open bolt system, and the semi-automatic function is simply the removal of the case, whereas an automatic weapon would both remove the empty case and chamber a fresh cartridge. See also self-loading rifle.

See also

• Semi-automatic pistol
• Semi-automatic rifle