The Gatling gun was the first successful machine gun. Unlike previous devices it was the first to combine reliability, high firing rate and ease of loading into a single device. It was designed by the American inventor Richard J. Gatling in 1861 and patented in 1862.
The design took advantage of the new brass bullet cartridge that had replaced the paper cartridge. The gun was a hand-driven rotary device, powered using a crank. A cylinder of ten barrels would be loaded and fired for each revolution. The loading was a simple gravity feed from a magazine mounted on top of the weapon. The maximum rate of fire was a possible 1,200 rounds per minute, although 400 was more reasonable. The gun was produced in calibres ranging from one inch down to 0.45 inch. It was operated by a crew of four.
The barrels, a carrier, and a lock cylinder were distinctly separate and all mounted on a solid plate revolving around a central shaft, mounted on a 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, 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 was then dropped to the ground.
The grouped barrel concept was not new, it had been tried since the 18th century, but poor engineering and the lack of a metal cartridge made the attempts unsuccessful. 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 cartridge holder was divided into sixteen secions each holding 25 cartridges. As each section was emptied the drum rotated bringing a new section into use until all 400 rounds had been fired.
The concept was made obsolete with the development of the gas or recoil blowback concept, which is the basis of modern machine guns.
However, Gatling-style guns with rotating barrels were to return as very high rate-of-fire weapons in military aircraft. One example is the six barrelled, 7,200 rpm M61 Vulcan.
Compare with the Montigny Mitrailleur, a contemporaneous Begium design using between 19 and 37 barrels.
(This image is from an 1885 encyclopedia in Swedish, http://www.lysator.liu.se/runeberg/nfai/0122.html )