The M61 and its derivatives have been the principal cannon armament of United States military fixed-wing aircraft for over sixty years.

[2] At the end of World War II, the United States Army Air Forces began to consider new directions for future military aircraft guns.

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.

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.

[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).

[6] 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.

A disadvantage of the M61 is that the bulk of the weapon, its feed system, and ammunition drum make it difficult to fit it into a densely packed airframe.

[14] A lighter version of the Vulcan developed for use on the F-22 Raptor, designated M61A2, is mechanically the same as the M61A1, but with thinner barrels to reduce overall weight to 92 kilograms (202 lb).

[16] Practically no powered rotary cannon is supplied with sufficient ammunition for a full minute of firing, due to its weight (at 6,000 rpm, the projectiles alone would represent a mass of about 600 kg (1,300 lb) for one minute of firing; and by including the brass shell, filling and primer the weight is slightly double that at 1,225 kg (2,701 lb)).

When vehicle-mounted, the only limiting factor is the vehicle's safe carry weight, so commensurately larger ammo storage is available.

Until the late 1980s, the M61 primarily used the M50 series of ammunition in various types, typically firing a 99-gram (3.5 oz) projectile at a muzzle velocity of about 1,030 metres per second (3,380 ft/s).

[18] The PGU-28/B is a "low-drag" round designed to reduce in-flight drag and deceleration, and has a slightly increased muzzle velocity of 1,050 metres per second (3,450 ft/s).

[21] The main types of combat rounds and their main characteristics are listed in the table: The Vulcan was first used in aerial combat on 4 April 1965, when four North Vietnamese Vietnam People's Air Force (VPAF) MiG-17s)[24] attacked a force of 10 North American F-100 Super Sabres (two of which were assigned weather reconnaissance duties) escorting 48 Vulcan-armed and "bomb-laden" F-105 Thunderchiefs, shooting down two of the latter.

[31][32][33][34] The Vulcan was later fitted into the weapons bay of some Convair F-106 Delta Dart and General Dynamics F-111 Aardvark models.

It was fitted in a side-firing installation on the Fairchild AC-119 and some marks of the Lockheed AC-130 gunships, and was used in the tail turrets of both the Convair B-58 Hustler and Boeing B-52H Stratofortress bombers.

[citation needed] A variant with much shorter barrels, designated the M195, was also developed for use on the M35 Armament Subsystem as used on the AH-1G Cobra helicopter.

This variant fed from ammunition boxes fitted to the landing skid and was developed to provide the AH-1 helicopter with a longer-range suppressive fire system before the adoption of the M97 universal turret mounting the M197 cannon.