Vulcan in Roman Mythology is the son of Jupiter and Juno, and husband of Maia and Venus. He was god of fire and volcanoes, and the manufacturer of art, arms and armor for gods and heroes. His smithy was believed to be situated underneath Mount Aetna in Sicily. At the Vulcanalia festival, which was held on August 23, fish and small animals were thrown into a fire. Vulcan's analogue in Greek mythology is the god Hephaestus.
At one point it was suspected that there existed a small planet that orbited between Mercury and the Sun, proposed to explain perturbations in Mercury's orbit from the path predicted by Classical Mechanics. This hypothetical planet was known as Vulcan. Albert Einstein's Theory of Relativity later explained these perturbations without the need for a third body, however, and so the possible existence of the planet Vulcan was discounted.
The name was later reused for a fictional planet in Star Trek.
The GAU-4 20mm Vulcan (with two models, designated M61A1 and M61A2) is a hydraulically driven, 6 barreled, rotary action, air cooled, electrically fired gatling gun with selectable rates of fire of either 4000 or 6000 rounds per minute. It is mounted on F-104, F-105, later models of the F-106, F-111, F-4, B-58, F-15, F-16, F-22, F-14 and F/A-18 aircraft, and is used for short-range air-to-air combat with secondary usefulness in strafing ground targets. It was developed in 1946 by the United States Army Ordnance Research and Development Service, and manufactured by General Electric. 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 weapon's current contractor is General Dynamics Armament Systems.
External link: http://www.fas.org/man/dod-101/sys/ac/equip/m61.htm