autocannon

thumb|upright=1.2|US M242 Bushmaster 25 mm autocannon mounted on an [[M2 Bradley armoured fighting vehicle]]

An autocannon, automatic cannon, machine cannon or anti-tank machine gun, is a fully automatic gun that is capable of rapid-firing large-caliber ( or more) armour-piercing, explosive or incendiary shells, as opposed to the smaller-caliber kinetic projectiles (bullets) fired by a machine gun. Autocannons have a longer effective range and greater terminal performance than machine guns, due to the use of larger/heavier munitions (most often in the range of , but bigger calibers also exist), but are usually smaller than tank guns, howitzers, field guns, or other artillery. When used on its own, the word "autocannon" typically indicates a non-rotary weapon with a single barrel. When multiple rotating barrels are involved, such a weapon is referred to as a "rotary autocannon" or "rotary cannon". If it uses a single barrel with a rotating cylinder with multiple chambers, it is known as a "revolver autocannon" or "revolver cannon"; both of these systems are commonly used as aircraft guns and anti-aircraft guns.