Automatic cannon

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, aircraft-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).

Although capable of generating a high rate of fire, autocannons overheat 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 use ammunition too quickly. 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. Systems with multiple barrels can have rates of fire of 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 air-to-air combat and close air support attacks on ground targets, where the target dwell time is short and weapons are typically operated in brief bursts.

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