Machine gun

A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rifle cartridges in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute. The first machine gun was the Maxim Gun, invented by Sir Hiram Maxim in 1884.
In United States law, machine gun is a term of art for any fully-automatic firearm, and also for any component or part that will modify an existing firearm into a fully-automatic firearm.

History:

The Chinese had much success with creating a repeating crossbow; the most common model, the Zhuge Nu, better known in the West as the Chu-ko-nu, is typically attributed to 2nd and 3rd century strategist Zhuge Liang, who developed it for the Kingdom of Shu during the Three Kingdoms period. However, a buried library in the ancient state of Chu indicates that some sort of repeating crossbow had at the very least been designed in the 3rd century BC. Other multi-shot weapons have a long development, going back to the 1st century in the West, with some claiming there were plans for a multi-shot arrow by Hero of Alexandria. Leonardo Da Vinci devised plans for one in the 15th century.
Some of the earliest firearms and attempts at higher rates of fire and some machine-gun-like traits existed as early as the 16th century, when Fathullah Shirazi (c. 1582), a Persian-Indian engineer and polymath who worked for Akbar the Great in the Mughal Empire, invented a multi-barrel gun, which had multiple gun barrels that fired hand cannons loaded with gunpowder. He also invented a primitive autocannon.
However, it would not be until the mid-19th century that successful machine-gun designs came into existence. The key characteristic of modern machine guns, their relatively high rate of fire and more importantly machine (automatic) loading, came with the Model 1862 Gatling gun, which was adopted by the United States Navy. These weapons were still powered by hand; however, this changed with Hiram Maxim's idea of harnessing recoil energy to power reloading in his Maxim machine gun. Dr. Gatling also experimented with electric-motor-powered models; this externally powered machine reloading has seen use in modern weapons as well. The Vandenburg and Miltrailleuse volley (organ) gun concepts have been revived partially in the early 21st century in the form of electronically controlled, multibarreled volley guns. It is important to note that what exactly constitutes a machine gun, and whether volley guns are a type of machine gun, and to what extent some earlier types of devices are considered to be like machine guns, is a matter of debate in many cases and can vary depending which language and exact definition is used.
Future:

Conventional machine-gun development has been slowed by the fact that existing machine-gun designs are adequate for most purposes, although significant developments are taking place with regard to antiarmor and antimissile weapons.
Electronically controlled machine guns with ultrahigh rates of fire, like Metal Storm's weapons may see use in some applications, although current small-caliber weapons of this type have found little use: they are too light for anti-vehicle use, but too heavy (especially with the need to carry a tactically useful amount of ammunition) for individual soldiers. The trend towards higher reliability and lower mass for a given power will likely continue. Another example is the six barreled, 4000 round per minute, XM214 minigun "six pack" developed by General Electric. It has a complex power train and weighs 85 pounds, factors which may, in some circumstances, mitigate against its deployment.

Human interface:

The most common interface on machine guns is a pistol grip and trigger. On earlier manual machine guns, the most common type was a hand crank. On externally powered machine guns, such as miniguns, an electronic button or trigger on a joystick is commonly used. Light machine guns often have a butt stock attached, while vehicle and tripod mounted machine guns usually have spade grips. In the late 20th century, scopes and other complex optics became more common as opposed to the more basic iron sights.
Loading systems in early manual machine guns were often from a hopper of loose (un-linked) cartridges. Manual-operated volley guns usually had to be reloaded manually all at once (each barrel reloaded by hand). With hoppers, the rounds could often be added while the weapon was firing. This gradually changed to belt-fed types. Belts were either held in the open by the person, or in a bag or box. Some modern vehicle machine guns used linkless feed systems however.

Closeup of M2 - This machine gun is part complex armament subsystem; it is aimed and fired from the aircraft rather than directly
Modern machine guns are usually mounted in one of four ways. The first is a bipod - often these are integrated with the weapon. This is common on light machine guns and also medium machine guns. Another major way is with a larger tripod, where the person holding it does not form a 'leg' of support. Medium and heavy machine guns usually use tripods. On ships and aircraft machine guns are usually mounted on a pintle mount - basically a steel post that is connected to the frame. Tripod and pintle mounts are usually used with spade grips. The last major mounting type is one that is disconnected from humans, as part of an armament system, such as a tank coaxial or part of aircraft's armament. These are usually electrically-fired and have complex sighting systems. (For examples see US Helicopter Armament Subsystems). Also there is a Qaud-pod style machine gun.