Hiram Maxim made guns reload themselves using their own recoil
The Gatling gun of 1862 fired fast, but a person still had to turn its crank. Hiram Maxim's breakthrough was to capture each shot's recoil energy and use it to reload, creating a weapon that keeps firing as long as the trigger is held. That principle defines the machine gun today.
By modern definition, a machine gun loads, fires and ejects continuously while the trigger is held back, until ammunition runs out or the trigger is released. That excludes hand-cranked Gatlings and volley guns such as the Nordenfelt. Weapons of 20 millimetres or larger count as autocannons, and today heavy machine gun means a calibre from 12.7 millimetres up to that limit, although the term began in World War I for especially weighty medium guns.
Attempts at rapid fire go back centuries. Medieval organ guns bundled barrels together, and in the mid-1570s an English engine of war tested at the Tower of London could reportedly fire up to about three times as fast as an arquebusier, though the government never adopted it. Revolving guns appeared in Nuremberg around 1580, and a 17th-century Chinese military text describes a hopper-fed string-of-100-bullets cannon that fired its charges one after another.
Other schemes stayed on paper or in doubt. In 1655 the Marquess of Worcester described a way of firing six wall muskets for 360 shots per minute, though it may never have been built. A 1663 paper to the Royal Society about a recoil- or gas-operated gun has never been found. In 1711 a French lawyer, Barbuot, showed the parliament of Dijon a crank-operated machine of ten carbine barrels, even proposing bellows to clear the smoke.
Heavy machine guns are usually mounted on tripods or vehicles, with extra crew to carry barrels and ammunition. Some are remarkably accurate: during the Vietnam War, Carlos Hathcock fitted a telescopic sight to a .50 calibre gun and hit a target at 2,250 metres, which inspired anti-materiel sniper rifles such as the Barrett M82. On fighter aircraft, rotary cannons have largely replaced true machine guns.
Source: Machine gun
