High Quality Content by WIKIPEDIA articles! An autocannon or automatic cannon is a rapid-fire projectile weapon firing a shell as opposed to the bullet fired by a machine gun. Autocannons often have a larger caliber than a machine gun (i.e., 20 mm or greater). Usually, autocannons are smaller than a field gun or other artillery, and are mechanically loaded for a faster rate of fire. They can use a variety of ammunition: common shells include high-explosive dual-purpose types (HEDP), any variety of armour-piercing (AP) types, such as composite rigid (APCR) or discarding sabot types (APDS).