A portable weapon that releases projectiles through a barrel by means of an explosive force. Firearms are identified by the internal diameter (bore) of the barrel, known as the calibre, and given in millimetres or less commonly inches, or in shotguns as a number designating gauge or bore (e.g. 12-gauge or 12-bore), and by their type. Types include handguns (revolvers, pistols), rifles, shotguns, carbines, machine guns, and submachine guns. A single-shot firearm must be reloaded after each discharge, a semi-automatic firearm performs the steps needed to prepare it to fire after each discharge but the trigger must be pressed for each discharge, and an automatic weapon continues discharging as long as the trigger remains depressed. Machine guns are fully automatic, as are smaller, lower-calibre submachine guns.