An autocannon, automatic cannon or machine cannon is a fully automatic gun that is capable of rapid-firing large-caliber (20 mm/0.79 in 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 20–57 mm (0.79–2.24 in), 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 simply "rotary cannon".
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Autocannons are heavy weapons that are too cumbersome for infantry. Due to the heavy weight and recoil, they are typically installed on fixed mounts, wheeled carriages, combat vehicles, aircraft or watercraft, and are almost always crew-served during operation, or even remote-operated with automatic target recognition/acquisition (e.g. sentry guns and naval CIWS). As such, ammunition is typically fed from a belt system to reduce reloading pauses or for a faster rate of fire, but magazines remain an option. Common types of ammunition, among a wide variety, include HEIAP, HEDP and more specialised armour-piercing (AP) munitions, mainly composite rigid (APCR) and discarding sabot (APDS) rounds.
Although capable of generating extremely rapid firepower, autocannons overheat quickly if used for sustained fire, and are limited by the amount of ammunition that can be carried by the weapons systems mounting them. Both the US 25 mm Bushmaster and the British 30 mm RARDEN have relatively slow rates of fire so as not to deplete ammunition too quickly. The Oerlikon KBA 25mm has a relatively mid-high rate of fire 650 rounds per minute but can be electronically programmed to 175-200 rounds per minute. The rate of fire of a modern autocannon ranges from 90 rounds per minute, in the case of the British RARDEN, to 2,500 rounds per minute with the GIAT 30. Rotary systems with multiple barrels can achieve over 10,000 rounds per minute (the Russian GSh-6-23, for example). Such extremely high rates of fire are effectively employed by aircraft in aerial dogfights and close air support on ground targets via strafing attacks, where the target dwell time is short and weapons are typically operated in brief bursts.