Home » Gun Information » Gun Glossary | Gun Terminology » Submachine Gun Submachine Gun:A submachine gun is a fully automatic (full-auto) firearm that fires pistol ammunition. These firearms are most effective for close-range combat.Some submachine guns are also capable of semi-automatic fire. Share:Click to share on Facebook (Opens in new window)Click to share on Twitter (Opens in new window)Click to share on Pinterest (Opens in new window)