The M61A1 Vulcan cannon is a six-barrel 20mm gun capable of
firing 6,000 rounds per minute. Its operation is based
upon the principle used in the rapid-firing gun invented by
Richard J. Gatling in the 1860s. The six rotating barrels,
firing one at a time, permit a high rate of fire while at the
same time reducing the problem of barrel wear and heat generation.
The gun can be driven electrically, hydraulically, or by a
ram-air turbine. In the F16 it is hydraulically driven. The Vulcan has equipped such USAF aircraft as the F-104, F-105B/D/F, F-15, F-16, A-7D, F-111A, F-4E, B-58, and B-52H.