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There have been two main machine gun eras: the era of manual machine guns and the era of automatic machine guns. The technical development itself is marked by a series of developments of specific automatic features, as well as technical developments (such as linked ammunition). The era of manual multi-shot devices extends back hundreds of years (such as manual volley guns), but the development of manual and automatic machine guns takes place almost entirely in the latter half of the 1800s. Manual machine guns are manually-powered, e.g., a crank must be turned to power reloading and firing, as opposed to simply holding down a trigger, as with automatic machine guns. There are many other notable features, but this is one of the most significant to allowing higher rates of fire common to machine guns.

Manual machine guns, as well as manual volley guns, saw their first major use in the American Civil War. The Gatling gun and “coffee gun” both used manually-powered automatic loading, fed via a hopper filled with cartridges. The Gatling gun would be the major type of the late 19th century, though there were many other manual designs with varying degrees of use (e.g. the Nordenfelt machine gun). The first automatic machine gun was the recoil-operated Maxim gun, which used linked (belt) ammunition, as well as a single barrel and automatic loading. This concept of using bullet energy would also drive the development of nearly all other semi and fully automatic firearms of 20th century.

The two major operation systems of modern automatic machine guns are gas operation and recoil operation. As the name implies, the gas operated system uses the gas generated from the burning powder to cycle the action, whereas the recoil operated uses the recoil generated from the ejecting bullet. The first gas-operated machine gun was the Colt-Browning M1895.[2]

Another (minor) type is the externally-powered machine gun. Rather than human manual power or energy generated by the cartridge, an external source such as an electric motor is used. These types are now called by more specific names such as Minigun and Chaingun. They are common on fighting aircraft and ground vehicles, where the externally powered mechanism allows for automatic clearing of many failure conditions that would otherwise disable the firearm.

[edit] Caliber Overview

Machine guns are generally categorized machine guns and autocannons. The separation takes place by caliber at about 20mm, with the larger-caliber guns being referred to as autocannons.

Another factor is whether the gun fires conventional rounds or explosive rounds. Guns firing large-caliber explosive rounds are generally either autocannons or automatic grenade launchers (“grenade machine guns”). Machine guns tend to share a very high ratio of barrel length to caliber (a long barrel for a small caliber).

[edit] Overview of modern automatic machine guns

Jędrusie Polish underground group firing a belt-fed water-cooled automatic machine gun- a Browning M1917 clone

Jędrusie Polish underground group firing a belt-fed water-cooled automatic machine gun- a Browning M1917 clone

The Gatling gun of a USAF A-10 ground attack aircraft.

The Gatling gun of a USAF A-10 ground attack aircraft.

Unlike semi-automatic firearms, which require one trigger pull per bullet fired, a machine gun is designed to fire bullets as long as the trigger is held down and ammunition is fed into the weapon. Although the term “machine gun” is often used by civilians to describe all fully automatic weapons, in military usage the term is restricted to relatively heavy weapons fired from some sort of support rather than hand-held, able to provide continuous or frequent bursts of automatic fire for as long as ammunition lasts. Machine guns are normally used against unprotected or lightly-protected personnel, or to provide suppressive fire.

Some machine guns have in practice maintained suppressive fire almost continuously for hours; other automatic weapons overheat after less than a minute of use. Because they become very hot, practically all machine guns fire from an open bolt, to permit air cooling from the breech between bursts. They also have either a barrel cooling system, or removable barrels which allow a hot barrel to be replaced.

Although subdivided into “light“, “medium“, “heavy” or “general purpose“, even the lightest machine guns tend to be substantially larger and heavier than other automatic weapons. Squad automatic weapons (SAWs) are a variation of light machine gun and only require one operator (sometimes with an assistant to carry ammunition). Medium and heavy machine guns are either mounted on a tripod or on a vehicle; when carried on foot, the machine gun and associated equipment (tripod, ammunition, spare barrels) require additional crew members.

The majority of machine guns are belt-fed, although some light machine guns are fed from drum or box magazines, and some vehicle-mounted machine guns are hopper-fed.

Other automatic weapons are subdivided into several categories based on the size of the bullet used, and whether the cartridge is fired from a positively locked closed bolt, or a non-positively locked open bolt. Fully automatic firearms using pistol-caliber ammunition are called machine pistols or submachine guns largely on the basis of size. Selective fire rifles firing a full-power rifle cartridge from a closed bolt are called automatic rifles, while those using a reduced-power rifle cartridge are called assault rifles.

Assault rifles are a compromise between the pistol-caliber submachine gun and a traditional rifle firing a full-power cartridge, allowing semi-automatic, burst and full-automatic fire options (selective fire). The modern legal definition of “assault rifle” is of significance in states like California, where according to state law, certain short, small-caliber, semi-automatic weapons are categorized as “assault weapons“, which were also made illegal by civilians to acquire or own. Supporters of gun rights generally consider the use of the phrase “assault weapon” to be pejorative when used to describe these civilian firearms, and this term is seldom used outside of the United States in this context.

The machine gun’s primary role in modern ground combat is to provide suppressive fire on an opposing force’s position, forcing the enemy to take cover and reducing the effectiveness of his fire [citation needed]. This either halts an enemy attack or allows friendly forces to attack enemy positions with less risk.

Light machine guns usually have simple iron sights. A common aiming system is to alternate solid (“ball”) rounds and tracer ammunition rounds (usually one tracer round for every four ball rounds), so shooters can see the trajectory and “walk” the fire into the target, and direct the fire of other soldiers.

Many heavy machine guns, such as the Browning M2 .50 caliber machine gun, are accurate enough to engage targets at great distances. During the Vietnam War, Carlos Hathcock set the record for a long-distance shot at 7382 ft (2250 m) with a .50 caliber heavy machine gun he had equipped with a telescopic sight [citation needed]. This led to the introduction of .50 caliber anti-material sniper rifles, such as the Barrett M82.

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