A firearm is said to fire from an open bolt or open breech if, when ready to fire, the bolt and working parts are held to the rear of the receiver, with no round in the chamber. When the trigger is actuated, the bolt travels forward, feeds a cartridge from the magazine or belt into the chamber, and fires that cartridge in the same movement. Like any other self-loading design, the action is cycled by the energy released from the propellant, which sends the bolt back to the rear, compressing the mainspring in readiness for firing the next round. In an open-bolt gun firing semi-automatically, the bolt is caught and held at this point by the sear after each shot; and in automatic open-bolt fire, it's caught and held in this manner whenever the trigger is released. In contrast to this, in closed-bolt guns, the trigger and sear do not affect the movement of the bolt directly.

A semi-automatic firearm, also called a self-loading or autoloading firearm, is a repeating firearm whose action mechanism automatically loads a following round of cartridge into the chamber and prepares it for subsequent firing, but requires the shooter to manually actuate the trigger in order to discharge each shot. Typically, this involves the weapon's action utilizing the excess energy released during the preceding shot to unlock and move the bolt, extracting and ejecting the spent cartridge case from the chamber, re-cocking the firing mechanism, and loading a new cartridge into the firing chamber, all without input from the user. To fire again, however, the user must actively release the trigger, allow it to "reset", before pulling the trigger again to fire off the next round. As a result, each trigger pull only discharges a single round from a semi-automatic weapon, as opposed to a fully automatic weapon, which will shoot continuously as long as the ammunition is replete and the trigger is kept depressed.