The M61 Vulcan (GAU-4 in USAF parlance) is a 20x102mm hydraulically or pneumatically powered Gatling-type rotary canon developed in the United States. Initially conceived as an aircraft mounted weapon, it is used by all branches of the American military in some form.
Using multiple barrels enables the weapon to develop a very high rate of fire (3,000+ rpm) without excessive wear or over-heating. The M61 was developed during the early days of jet combat where it was determined that, due to the higher speeds encountered, it was desirable to have a weapon which fired extremely fast and with a projectile containing an explosive payload. Fixed-wing aircraft usually mount the M61 as an integral part of the fuselage, although gun pods have been used (notably the F-4 Phantom II). Rotary-wing craft have traditionally mounted the M61 in pods on pylons. The US Army developed a crew-served, gas-operated version known as the M134 that operated under its own power foregoing the need for an external source.