An autocannon or automatic cannon is a large, fully automatic, rapid-fire projectile weapon that fires armour-piercing or explosive shells, as opposed to the bullet fired by a machine gun. Autocannons often have a larger calibre than a machine gun (e.g., 20 mm or greater), but are usually smaller than a field gun or other artillery. When used on its own, the word "autocannon" indicates a single barrel weapon. When multiple rotating barrels are involved, the word "rotary" is added, and such a weapon is referred to as a "rotary autocannon." Modern autocannons are typically not single soldier-portable or stand-alone units, rather they are usually vehicle-mounted, plane mounted, or boat-mounted, or even remote-operated as in some naval applications. As such, ammunition is typically fed from a belt to reduce reloading or for a faster rate of fire, but a magazine remains an option. 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).