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A multiple barrel firearm is a firearm of any type with more than one barrel, usually to increase the rate of fire/hitting probability and to reduce barrel erosion/overheating.
Multiple barrel firearms date back to the 14th century, when the first primitive volley guns were developed. They are made with several barrels for firing a number of shots, either simultaneously or in succession. They differ from modern machine guns in that they lack automatic loading and automatic fire and are limited by the number of barrels bundled together.
In practice the large ones were not particularly more useful than a cannon firing canister shot or grapeshot. Since they were still mounted on a carriage, they could be as hard to aim and move around as a cannon, and the many barrels took as long or longer to reload. They also tended to be relatively expensive since they were more complex than a cannon, due to all the barrels and ignition fuses, and each barrel had to be individually maintained and cleaned.
The Gatling gun is one of the best-known early rapid-fire spring loaded, hand cranked weapons and a forerunner of the modern machine gun and rotary cannon. Invented by Richard Gatling, it saw occasional use by the Union forces during the American Civil War in the 1860s, which was the first time it was employed in combat. Later, it was used again in numerous military conflicts, such as the Boshin War, the Anglo-Zulu War, and the assault on San Juan Hill during the Spanish-American War. It was also used by the Pennsylvania militia in episodes of the Great Railroad Strike of 1877, specifically in Pittsburgh.
The Gatling gun's operation centered on a cyclic multi-barrel design which facilitated cooling and synchronized the firing-reloading sequence. Each barrel fired a single shot when it reached a certain point in the cycle, after which it ejected the spent cartridge, loaded a new round, and, in the process, allowed the barrel to cool somewhat. This configuration allowed higher rates of fire to be achieved without the barrels overheating.
Gatling later replaced the hand-cranked mechanism of a rifle-caliber Gatling gun with an electric motor, a relatively new invention at the time. Even after Gatling slowed the mechanism, the new electric-powered Gatling gun had a theoretical rate of fire of 3,000 rounds per minute, roughly three times the rate of a typical modern, single-barreled machine gun. Gatling's electric-powered design received U.S. Patent #502,185 on July 25, 1893. Despite Gatling's improvements, the Gatling gun fell into disuse after cheaper, lighter-weight, recoil and gas operated machine guns were invented; Gatling himself went bankrupt for a period.
During World War I, several German companies were working on externally powered guns for use in aircraft. Of those, the best-known today is perhaps the Fokker-Leimberger, an externally powered 12-barrel rotary gun using the 7.92×57mm Mauser round; it was claimed to be capable of firing over 7,000 rpm, but suffered from frequent cartridge-case ruptures due to its "nutcracker", rotary split-breech design, which is fairly different from that of a Gatling. None of these German guns went into production during the war, although a competing Siemens prototype (possibly using a different action) which was tried on the Western Front scored a victory in aerial combat. The British also experimented with this type of split-breech during the 1950s, but they were also unsuccessful.
In the 1960s, the United States Armed Forces began exploring modern variants of the electric-powered, rotating barrel Gatling-style weapons for use in the Vietnam War. American forces in the Vietnam War, which used helicopters as one of the primary means of transporting soldiers and equipment through the dense jungle, found that the thin-skinned helicopters were very vulnerable to small arms fire and rocket-propelled grenade (RPG) attacks when they slowed to land. Although helicopters had mounted single-barrel machine guns, using them to repel attackers hidden in the dense jungle foliage often led to barrels overheating or cartridge jams.
In order to develop a weapon with a more reliable, higher rate of fire, General Electric designers scaled down the rotating-barrel 20 mm M61 Vulcan cannon for 7.62×51mm NATO ammunition. The resulting weapon, the M134 Minigun, could fire up to 6,000 rounds per minute without overheating. The gun has a variable, i.e., selectable, rate of fire specified to fire at rates of up to 6,000 rpm, with most applications set at rates between 3,000-4,000 rounds per minute.
The Minigun was mounted on Hughes OH-6 Cayuse and Bell OH-58 Kiowa side pods; in the turret and on pylon pods of Bell AH-1 Cobra attack helicopters; and on door, pylon and pod mounts on Bell UH-1 Iroquois transport helicopters. Several larger aircraft were outfitted with miniguns specifically for close air support: the Cessna A-37 Dragonfly with an internal gun and with pods on wing hardpoints; and the Douglas A-1 Skyraider, also with pods on wing hardpoints. Other famous gunship airplanes were the Douglas AC-47 Spooky, the Fairchild AC-119, and the Lockheed AC-130.