The Gatling gun (1861) was one of the most well known rapid-fire weapons to be used in the 1860s by the Union forces of the American Civil War, following the 1851 invention of the mitrailleuse by the Belgian Army.
Although the first Gatling gun was capable of firing continuously, it required human power to crank it; as such it was not a true automatic weapon. Each barrel fired a single shot as it reached a certain point in the cycle after which it ejected the spent cartridge, loaded a new round, and in the process, cooled down somewhat.
This configuration allowed higher rates of fire without the problem of an overheating single barrel. Some time later, Gatling-type weapons were invented that diverted a fraction of gas from the chamber to turn the barrels. Later still, electric motors supplied external power.
The original Gatling gun was designed by the American inventor Dr. Richard J. Gatling in 1861 and patented in 1862. He wrote that he made it to reduce the size of armies and so reduce the number of deaths by combat and disease.
The Maxim gun, invented in 1884, was the first true automatic weapon, making use of the fired projectile's recoil force to reload the weapon.
Patent drawing for R.J. Gatling's Battery Gun, 9 May 1865.
The original Gatling gun was a field weapon, which used multiple rotating barrels turned by a hand crank, and firing loose (no links or belt) metal cartridge ammunition using a gravity feed system from a hopper. The Gatling gun's innovation lay neither in the rotating mechanism (featured by many revolvers of the day) nor the use of multiple barrels to limit overheating (used by the mitrailleuse gun); rather, the innovation was the gravity feed reloading mechanism, which allowed unskilled operators to achieve a relatively high rate of fire of 200 rounds per minute.
The Gatling gun was first used in warfare during the American Civil War. The gun was not accepted by the Union Army until 1866, but a "sales engineer" of the manufacturing company demonstrated it in combat. Lieutenant A.L. Howard of the Connecticut National Guard had an interest in the company manufacturing Gatling guns, and took a personally-owned Gatling gun to Saskatchewan in Canada in 1885 for use with the Canadian military against the Métis during Louis Riel's North-West Rebellion.
Early multi-barrel guns were approximately the size and weight of artillery pieces, and were often perceived as a replacement for cannon firing grapeshot or cannister shot. Unlike earlier weapons such as the Mitrailleuse which required manual reloading, the Gatling gun was more reliable, easier to operate, and had a higher firing rate. The large wheels required to move these guns around required a high firing position which increased the vulnerability of their crews. Sustained firing of gunpowder cartridges generated a cloud of smoke making concealment impossible until Smokeless powder became available in the late 19th century. When fighting troops of industrialized nations, Gatling guns could be targeted by artillery they could not reach and their crews could be targeted by snipers they could not see.
The Gatling gun was used most successfully to expand European colonial empires by killing warriors of non-industrialized societies including the Matabele, the Zulu, the Bedouins, and the Mahdists. Imperial Russia purchased 400 Gatling guns and used them against Turcoman cavalry and other nomads of central Asia. The Royal Navy used Gatling guns against the Egyptians at Alexandria in 1882.
Gatling guns were used by the US side during the Spanish-American War, most notably during the battle of San Juan Hill.
 Basic design of the original gun
A British 1865 Gatling gun at Firepower - The Royal Artillery Museum
The Gatling gun was hand-crank operated with six barrels revolving around a central shaft, similar to the Puckle Gun. Early models had a fibrous matting stuffed in among the barrels which could be soaked with water to cool the barrels down. Later models eliminated the matting-filled barrels as being counterproductive. The ammunition was initially a steel cylinder charged with black powder and primed with a percussion cap, because self-contained brass cartridges had not yet been invented. The shells were gravity-fed into the breech through a hopper or stick magazine on top of the gun. Each barrel had its own firing mechanism. After 1861, new brass cartridges similar to modern cartridges replaced the paper cartridge, but Gatling did not switch to them immediately.
The model of 1881 was designed to use the 'Bruce'-style feed system (U.S. Patents 247,158 and 343,532) that accepted two rows of .45/70 cartridges. While one row was being fed into the gun, the other could be reloaded, thus allowing sustained fire. The final gun required four operators. By 1876 the Gatling gun had a theoretical rate of fire of 1,200 rounds per minute, although 400 rounds per minute was more readily achievable in combat.
Each barrel fires once per revolution at about the same position. The barrels, a carrier, and a lock cylinder were separate and all mounted on a solid plate revolving around a central shaft, mounted on an oblong fixed frame. The carrier was grooved and the lock cylinder was drilled with holes corresponding to the barrels. Each barrel had a single lock, working in the lock cylinder on a line with the barrel. The lock cylinder was encased and joined to the frame. The casing was partitioned, and through this opening the barrel shaft was journaled. In front of the casing was a cam with spiral surfaces. The cam imparted a reciprocating motion to the locks when the gun rotated. Also in the casing was a cocking ring with projections to cock and fire the gun.
Turning the crank rotated the shaft. Cartridges, held in a hopper, dropped individually into the grooves of the carrier. The lock was simultaneously forced by the cam to move forward and load the cartridge, and when the cam was at its highest point, the cocking ring freed the lock and fired the cartridge. After the cartridge was fired the continuing action of the cam drew back the lock bringing with it the spent cartridge which then dropped to the ground.
The grouped barrel concept had been explored by inventors since the 18th century, but poor engineering and the lack of a unitary cartridge made previous designs unsuccessful. The initial Gatling gun design used self-contained, reloadable steel cylinders with a chamber holding a ball and black-powder charge, and a percussion cap nipple on one end. As the barrels rotated, these steel cylinders dropped into place, were fired, and were then ejected from the gun. The innovative features of the Gatling gun were its independent firing mechanism for each barrel and the simultaneous action of the locks, barrels, carrier and breech.
The smallest calibre gun also had a Broadwell drum feed in place of the curved magazine of the other guns. The drum, named after L. W. Broadwell, an agent for Gatling's company, comprised twenty stick magazines arranged around a central axis, like the spokes of a wheel, each holding twenty cartridges with the bullet noses oriented toward the central axis. This significant invention does not appear to have been patented separately, and may have been included in the April 9, 1872 patent, U.S. 125,563; a post and base, apparently for mounting a Broadwell drum, is visible in Figure 13 of U.S. 125,563. As each magazine emptied, the drum was manually rotated to bring a new magazine into use until all 400 rounds had been fired.
 Modern Gatling-style guns
The GAU-8 Gatling gun of an A-10 Thunderbolt II at Osan Air Base, South Korea.
After Gatling guns were replaced by lighter, cheaper blowback style weapons, the approach of using multiple rotating barrels fell into disuse for many decades. However, Gatling gun-style weapons made a return in the 1940–50s, when weapons with very high rates of fire were needed in military aircraft. For these modern weapons, electric motors are used to rotate the barrel, although systems that derive power from their ammunition do exist such as the GShG-7.62 machine gun and GSh-6-23, which uses a gas-operated drive system.
One of the main reasons for the resurgence of the Gatling gun-style design is the weapon's tolerance for continuous high rates of fire. For example, if 500 rounds were fired per minute from a conventional single-barrel weapon, this would likely result in the barrel overheating (distorting in extreme cases) or a weapon jam. In contrast, a five-barreled Gatling gun-style weapon firing 500 rounds, only fires 100 rounds per barrel, an acceptable rate of fire. Ultimately the limiting factor is the rate at which loading and extraction can occur. In a single barrel design these tasks must alternate, a multiple barrel design on the other hand lets them occur simultaneously, with different barrels at different points in the cycle. Their high rate of fire also makes them useful in systems that have little time to engage their targets, such as CIWS which defend against fast-moving anti-ship missiles.
The M61 Vulcan 20 mm cannon is the most prolific member of a family of weapons designed by General Electric and currently manufactured by General Dynamics. It is a six-barreled rotary cannon capable of more than 6,000 rounds per minute. Similar systems are available ranging from 5.56 mm to 30 mm (there was even a 37 mm Gatling on the prototype T249 Vigilante AA platform); the rate-of-fire being somewhat inversely-proportional to the size and mass of the ammunition (which also determines the size and mass of the barrels).
Another Gatling design well-known among aviation enthusiasts is the GAU-8 Avenger 30 mm cannon, carried on the A-10 Thunderbolt II (Warthog) attack aircraft. It is a seven-barreled cannon designed for tank-killing and is currently the largest bore Gatling weapon active in the U.S. arsenal.
During the Vietnam War, the 7.62 mm caliber M134 Minigun was created as a helicopter weapon. Able to fire 6,000 rounds per minute from a 4,000-round linked belt, the Minigun proved to be one of the most effective non-explosive projectile weapons ever built and is still used in helicopters today.
They are also used on USAF AC-47, AC-119 and Lockheed AC-130 gunships, their original high-capacity cargo airframes able to house the items needed for sustained operation. With sophisticated navigation and target identification tools, Miniguns can be used effectively even against concealed targets. The crew's ability to concentrate the Gatling's fire very tightly produces the appearance of the 'Red Tornado' from the light of the tracers, as the gun platform circles a target at night.