Autocannon
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An autocannon is a projectile weapon with a larger caliber (bore) than a machine gun, but smaller than field gun or other artillery. They have mechanisms to automatically load the ammunition and have a faster rate of fire than artillery.
The term cannon was used during WWII to describe guns used in aircraft, where the distinction was that the shells were explosive. After the war similar guns were used with non-explosive rounds in the anti-tank role, and the name autocannon started to become popular. Autocannon today are typically distinguished by their incorporation of some method of automated loading and firing.
Examples of an autocannon are the 25 mm M242 Bushmaster mounted on the M2 Bradley, the Bofors 40 mm anti-aircraft gun, the Mauser BK-27, which is used in all aircraft and on all navy vessels of the Bundeswehr and also by the US Navy, or the 30 mm GAU-8 in the A-10 Warthog close air support airframe.
Autoloading equipment had been used on anti-aircraft guns and for other guns (such as the Molins gun) during the Second World War. Modern tank guns (around 120 mm calibre) have been fitted with automatic loading systems (typically Soviet Union and Russian main battle tanks, along with French, have used these for reasons of space) and although they technically might be considered to be autocannon they are not referred to as such, due to their relatively low cadence. A notable exception to this might be the Russian AK-130 130mm/70 twin naval gun, used on most large Russian warships, which has a fire rate in excess of 60 rounds per minute, currently the fastest-firing weapon in excess of 120mm known.