M61 Vulcan

The M61 Vulcan is a hydraulically, electrically, or pneumatically driven, six-barrel, air-cooled, electrically fired Gatling-style rotary cannon which fires 20 mm × 102 mm (0.787 in × 4.016 in) rounds at an extremely high rate (typically 6,000 rounds per minute). The M61 and its derivatives have been the principal cannon armament of United States military fixed-wing aircraft for over sixty years.

The M61 was originally produced by General Electric. After several mergers and acquisitions, it is produced by General Dynamics as of 2000^[update]. It is also manufactured under license in Japan by Sumitomo Heavy Industries for Japan's Self-Defense Force and by SNT Dynamics in South Korea.

At the end of World War II, the United States Army Air Forces began to consider new directions for future military aircraft guns. The higher speeds of jet-powered fighter aircraft meant that achieving an effective number of hits would be extremely difficult without a much higher volume of fire. While captured German designs (principally the Mauser MG 213C) showed the potential of the single-barrel revolver cannon, the practical rate of fire of such a design was still limited by ammunition feed and barrel wear concerns. The Army wanted something more advanced, combining an extremely high rate of fire with exceptional reliability.

In 1947, the Air Force became a separate branch of the military. The new Air Force made a request for a new aircraft gun. A lesson of World War II air combat was that German, Italian, and Japanese fighters could attack American aircraft from long range with their cannon main armament. American fighters with .50 caliber (12.7 mm) main armament, such as the P-51 and P-47, had to be close to enemy aircraft in order to hit and damage them. The 20 mm (0.79 in) Hispano cannon carried by the P-38 and P-61, while formidable against propeller-driven planes, had a relatively low rate of fire in the age of jets, while other cannons were notoriously unreliable.

In response to this requirement, the Armament Division of General Electric resurrected an old idea: the multi-barrel Gatling gun. The original Gatling gun had fallen out of favor because of the need for an external power source to rotate the barrel assembly, but the new generation of turbojet-powered fighters offered sufficient electric power to operate the gun, and electric operation was more reliable than gas-operated reloading.

With multiple barrels, the rate of fire per barrel could be lower than a single-barrel revolver cannon while providing a greater overall rate of fire. The idea of powering a Gatling gun from an external electric power source was not a novel idea at the end of World War II, as Richard Jordan Gatling himself had done just that with a patent he filed in 1893.

During World War I, a similar 12-barreled Fokker-Leimberger aircraft rotary machine gun, powered by either the aircraft engine or an electric motor, had been under development by the German Empire.^{[citation needed]}

In 1946, the Army issued General Electric a contract for "Project Vulcan", a six-barrel weapon capable of firing 7,200 rounds per minute (rpm). Although European designers were moving towards heavier 30 mm (1.181 in) weapons for better hitting power, the U.S. initially concentrated on a powerful 0.60-inch (15 mm) cartridge designed for a pre-war anti-tank rifle, expecting that the cartridge's high muzzle velocity would be beneficial for improving hit ratios on high-speed targets. The first GE prototypes of the 0.60-inch (15 mm) caliber T45 were ground-fired in 1949; it achieved 2,500 rpm, which was increased to 4,000 rpm by 1950. Due to air combat experience in the Korean War, the USAF decided that high velocity alone might not be sufficient to ensure target destruction and tested 20 and 27 mm (0.787 and 1.06 in) alternatives based on the 0.60-inch (15 mm) caliber cartridge. These variants of the T45 were known as the T171 and T150 respectively and were first tested in 1952. Eventually, the standard 20×102 mm cartridge was determined to have the desired balance of projectile/explosive mass and muzzle velocity, resulting in an optimum balance of range, accuracy and kinetic energy on target.