In June 1946, the General Electric Company was awarded the contract for "Project Vulcan." Rather than focusing on hitting power as European designers were doing with their slow-firing 30mm aircraft cannons, the project focused on a pre-war .60 caliber (15mm) anti-tank rifle cartridge, aiming for a rate of fire no less than 6,000 rounds per minute.
The early T45 model using the .60 caliber round had issues with insufficient damage, and alternatives in 20mm and 27mm were tested, the T171 and T150 guns.
In 1956 the T171 20mm gun was standardized by the US Army and US Air Force as the M61 20mm Vulcan aircraft gun.
The M61 Vulcan is an externally powered six-barrel rotary gun having a rate of fire of up to 7,200 rounds per minute.
The firing rate is selectable at 4,000 or 6,000 rounds per minute.
Each of the gun's six barrels fires only once during each revolution of the barrel cluster.
The six rotating barrels contribute to long weapon life by minimizing barrel erosion and heat generation.
The gun's rate of fire, essentially 100 rounds per second, gives the pilot a shot density that will enable a "kill" when fired in one-second bursts.
The gun fires electrically primed 20x102mm ammunition and usually uses a hydraulic motor for power, though there is a self-powered version, the GAU-4 (M130 in Army service) which was used in the SUU-23/A / M25 gunpod.
This variant uses an electric motor to spin up the barrel cluster, then sustains itself via gas operation.
While the initial M61 was troubled by issues with misfeeds and FOD damage to aircraft mounting it due to using linked ammunition, the linkless M61A1 Vulcan cannon is a proven gun, having been the US military's close-in weapon of choice dating back to 1959 when it was first fielded on the F-104C.
The F-104, F-105, X-32, F-14, later models of the F-106, F-111, F-4, B-47, B-52 (until the 1990s) and B-58 all used the M61, as do the Air Force's F-15, F-16 and F-22, and the Navy's F/A-18.