A machine gun is a fully automatic mounted or portable firearm designed to fire bullets in quick succession from an ammunition belt or magazine, typically at a rate of 300 to 1800 rounds per minute. Note that not all fully-automatic firearms are machine guns. Submachine guns, rifles, assault rifles, shotguns, pistols or cannons may be capable of fully automatic fire, but are not designed for sustained fire. As a class of military rapid-fire guns, true machine guns are fully automatic weapons designed to be used as support weapons and generally used when attached to a mount or fired from the ground on a bipod or tripod. Many (though by no means all) machine guns also use belt feeding and open bolt operation, features not normally found on rifles.
- Overview of modern machine guns
- Early rapid firing weapons
- Maxim and World War 1
- Interwar era and World War II
- Cold War
- Human interface
In United States gun law, machine gun is also a legal term for any weapon able to fire more than one shot per trigger pull regardless of caliber, the receiver of any such weapon, any weapon convertible to such a state using normal tools, or any component or part that will modify an existing firearm such that it functions as a "machine gun" such as a drop-in auto sear. Civilian possession of such weapons is not prohibited by any Federal law and not illegal in many states, but they must be registered as Title II weapons under the National Firearms Act and have a tax stamp paid. The Hughes Amendment to the Firearm Owners Protection Act of 1986 banned new production of firearms classified as machine guns for most civilian applications, however, so only "grandfathered" weapons produced before this date are legally transferable.
Overview of modern machine guns
Unlike semi-automatic firearms, which require one trigger pull per round fired (usually due to the sear only resetting when the trigger is released), a machine gun is designed to fire for as long as the trigger is held down. Nowadays the term is restricted to relatively heavy weapons, able to provide continuous or frequent bursts of automatic fire for as long as ammunition lasts. Machine guns are normally used against personnel, aircraft and light vehicles, or to provide suppressive fire, either directly or indirectly. They are commonly mounted on fast attack vehicles such as technicals to provide heavy mobile firepower, armored vehicles such as tanks for engaging targets too small to justify use of the primary weaponry or too fast to effectively engage with it, and on aircraft as defensive armament or for strafing ground targets, though on fighter aircraft true machine guns have mostly been supplanted by large-caliber rotary guns.
Some machine guns have in practice sustained 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 usually have either a barrel cooling system, slow-heating heavyweight barrel, 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 standard infantry arms. 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.
Light machine guns are designed to provide mobile fire support to a squad and are typically air-cooled weapons fitted with a box magazine or drum and a bipod; they may use full-size rifle rounds, but modern examples often use intermediate rounds. Medium machine guns use full-sized rifle rounds and are designed to be used from fixed positions mounted on a tripod. Heavy machine gun is a term originating in World War 1 to describe heavyweight medium machine guns and persisted into World War 2 with Japanese Hotchkiss M1914 clones; today, however, it is used to refer to automatic weapons with a caliber of at least .50in (12.7mm) but less than 20mm. A general-purpose machine gun is usually a lightweight medium machine gun which can either be used with a bipod and drum in the light machine gun role or a tripod and belt feed in the medium machine gun role.
US Army doctrine also includes the role of "squad automatic weapon" (SAW), a weapon which is used by a single crewman and is regarded as an "automatic rifle" rather than a machine gun, though the weapon itself may actually be a machine gun in functional terms. FM 3-22.68 "Crew-Served Machine Guns", describes how the M249 can be used either as a machine gun or as an automatic rifle: "Both the M249 automatic rifle and the M249 machine gun are identical, but its employment is different. The M249 automatic rifle is operated by an automatic rifleman, but its ammunition may be carried by other Soldiers within the squad or unit. The M249 machine gun is a crew-served weapon."
Machine guns usually have simple iron sights, though the use of optics is becoming more common. A common aiming system for direct fire 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. This led to the introduction of .50 caliber anti-materiel sniper rifles, such as the Barrett M82.
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; those using shotgun cartridges are almost always referred to as automatic shotguns. The term personal defense weapon (PDW) is sometimes applied to weapons firing dedicated armor-piercing rounds which would otherwise be regarded as machine pistols or SMGs, but it is not particularly strongly defined and has historically been used to describe a range of weapons from ordinary SMGs to compact assault rifles. Selective fire rifles firing a full-power rifle cartridge from a closed bolt are called automatic rifles or battle rifles, while rifles that fire an intermediate cartridge are called assault rifles.
Assault rifles are a compromise between the size and weight of a pistol-caliber submachine gun and a full size battle rifle, firing intermediate cartridges and allowing semi-automatic and burst or full-automatic fire options (selective fire), sometimes with both of the latter present.
In certain states, like California, certain weapons that cosmetically resemble true assault rifles, but are only semi-automatic (autoloading), are categorized as assault weapons and possession by civilians is generally illegal. Supporters of gun rights generally consider this application of the phrase "assault weapon" to be a misnomer and this term is in fact seldom used outside of the United States for these civilian firearms.
All machine guns follow a cycle:
The operation is basically the same for all autoloading firearms, regardless of the means of activating these mechanisms.
Most modern machine guns use gas-operated reloading. A recoil actuated machine gun uses the recoil to first unlock and then operate the action. Machine guns such as the M2 Browning and MG42, are of this type. A cam, lever or actuator demultiplicates the energy of the recoil to operate the bolt.
An externally actuated weapon uses an external power source, such as an electric motor or even a hand crank to move its mechanism through the firing sequence. Most modern weapons of this type are called Gatling guns in reference to their driving mechanism. Gatling guns have several barrels each with an associated action on a rotating carousel and a system of cams that load, cock, and fire each mechanism progressively as it rotates through the sequence; essentially each barrel is a separate bolt-action rifle using a common feed source. The continuous nature of the rotary action allows for an incredibly high cyclic rate of fire, often several thousand rounds per minute. Rotary guns are less prone to jamming than a gun operated by gas or recoil, as the external power source will eject misfired rounds with no further trouble, but this is not possible in the rare cases of self-powered rotary guns. Rotary guns are generally used with large rounds, 20 mm in diameter or more, offering benefits of reliability and firepower, though the weight and size of the power source and driving mechanism makes them impractical for use outside of a vehicle or aircraft mount.
Revolver cannons, such as the Mauser MK 213, were developed in World War II by the Germans to provide high-caliber cannons with a reasonable rate of fire and reliability. A recoil-operated carriage holds a revolving chamber with typically five chambers. As each round is fired, electrically, the carriage moves back rotating the chamber which also ejects the spent case, indexes the next live round to be fired with the barrel and loads the next round into the chamber. The action is very similar to that of the revolver pistols common in the 19th and 20th centuries, giving this type of weapon its name.
Firing a machine gun for prolonged periods produces large amounts of heat. In a worst-case scenario this may cause a cartridge to overheat and detonate even when the trigger is not pulled, potentially leading to damage or causing the gun to cycle its action and keep firing until it has exhausted its ammunition supply or jammed (this is known as cook-off, distinct from runaway fire where the sear fails to disengage when the trigger is released). To prevent this, some kind of cooling system is required. Early machine guns were often water-cooled; while very effective, the water also added considerable weight to an already bulky design. Air-cooled machine guns often feature quick-change barrels, often carried by a crew member, passive cooling fins, or in some designs systems of forced-air cooling such as that employed by the Lewis Gun. The higher the rate of fire, the more often barrels must be changed and allowed to cool. To minimize this, most air-cooled guns are fired only in short bursts or at a reduced rate of fire. Some designs - such as the many variants of the MG42 - are capable of rates of fire in excess of 1,200 rounds per minute.
In weapons where the round seats and fires at the same time, mechanical timing is essential for operator safety, to prevent the round from firing before it is seated properly. Machine guns are controlled by one or more mechanical sears. When a sear is in place, it effectively stops the bolt at some point in its range of motion. Some sears stop the bolt when it is locked to the rear. Other sears stop the firing pin from going forward after the round is locked into the chamber. Almost all machine guns have a "safety" sear, which simply keeps the trigger from engaging.
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 mechanical 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.
While technical use of the term "machine gun" has varied, the modern definition used by the Sporting Arms and Ammunition Manufacturers' Institute of America is "A fully automatic firearm that loads, fires and ejects continuously when the trigger is held to the rear until the ammunition is exhausted or pressure on the trigger is released." This definition excludes most early manually operated repeating arms such as volley guns and the Gatling gun.
Early rapid-firing weapons
The first known ancestors of multi-shot weapons were medieval organ guns, while the first to have the ability to fire multiple shots from a single barrel without a full manual reload were revolvers made in Europe in the late 1500s. One is a shoulder-gun-length weapon made in Nuremberg, Germany, circa 1580. Another is a revolving arquebus, produced by Hans Stopler of Nuremberg in 1597.
True repeating long arms were difficult to manufacture prior to the development of the unitary firearm cartridge; nevertheless, lever-action repeating rifles such as the Kalthoff repeater and Cookson repeater were made in small quantities in the 17th century.
Another early revolving gun was created by James Puckle, a London lawyer, who patented what he called "The Puckle Gun" on May 15, 1718. It was a design for a manually operated 1.25 in. (32 mm) caliber, flintlock cannon with a revolver cylinder able to fire 6-11 rounds before reloading by swapping out the cylinder, intended for use on ships. It was one of the earliest weapons to be referred to as a machine gun, being called such in 1722, though its operation does not match the modern usage of the term. According to Puckle, it was able to fire round bullets at Christians and square bullets at Turks. However, it was a commercial failure and was not adopted or produced in any meaningful quantity.
In 1777, Philadelphia gunsmith Joseph Belton offered the Continental Congress a "new improved gun", which was capable of firing up to twenty shots in five seconds; unlike older repeaters using complex lever-action mechanisms, it used a simpler system of superposed loads, and was loaded with a single large paper cartridge. Congress requested that Belton modify 100 flintlock muskets to fire eight shots in this manner, but rescinded the order when Belton's price proved too high.
In the early and mid-19th century, a number of rapid-firing weapons appeared which offered multi-shot fire, mostly volley guns. Volley guns (such as the Mitrailleuse) and double barreled pistols relied on duplicating all parts of the gun, though the Nock gun used the otherwise-undesirable "chain fire" phenomenon (where multiple chambers are ignited at once) to propagate a spark from a single flintlock mechanism to multiple barrels. Pepperbox pistols also did away with needing multiple hammers but used multiple manually operated barrels. Revolvers further reduced this to only needing a pre-prepared cylinder and linked advancing the cylinder to cocking the hammer. However, these were still manually operated.
The Agar Gun, otherwise known as a "coffee-mill gun" because of its resemblance to a coffee mill, was invented by Wilson Agar at the beginning of the US Civil War. The weapon featured mechanized loading using a hand crank linked to a hopper above the weapon. The weapon featured a single barrel and fired through the turning of the same crank; it operated using paper cartridges fitted with percussion caps and inserted into metal tubes which acted as chambers; it was therefore functionally similar to a revolver. The weapon was demonstrated to President Lincoln in 1861. He was so impressed with the weapon that he purchased 10 on the spot for $1,500 apiece. The Union Army eventually purchased a total of 54 of the weapons. However, due to antiquated views of the Ordnance Department the weapons, like its more famous counterpart the Gatling Gun, saw only limited use.
The Gatling gun, patented in 1861 by Richard Jordan Gatling, was the first to offer controlled, sequential fire with mechanical loading. The design's key features were machine loading of prepared cartridges and a hand-operated crank for sequential high-speed firing. It first saw very limited action in the American Civil War; it was subsequently improved and used in the Franco-Prussian war and North-West Rebellion. Many were sold to other armies in the late 19th century and continued to be used into the early 20th century, until they were gradually supplanted by Maxim guns. Early multi-barrel guns were approximately the size and weight of contemporary artillery pieces, and were often perceived as a replacement for cannon firing grapeshot or canister shot. 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. Gatling guns were targeted by artillery they could not reach and their crews were 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.
In 1870 a Lt. D. H. Friberg of the Swedish army patented a fully automatic recoil-operated firearm action and may have produced firing prototypes of a derived design around 1882: this was the forerunner to the 1907 Kjellman machine gun, though due to rapid residue buildup from the use of black powder Friberg's design was not a practical weapon.
Maxim and World War 1
The first practical self-powered machine gun was invented in 1884 by Sir Hiram Maxim. The "Maxim gun" used the recoil power of the previously fired bullet to reload rather than being hand-powered, enabling a much higher rate of fire than was possible using earlier designs such as the Nordenfelt and Gatling weapons. Maxim also introduced the use of water cooling, via a water jacket around the barrel, to reduce overheating. Maxim's gun was widely adopted and derivative designs were used on all sides during the First World War. The design required fewer crew and was lighter and more usable than the Nordenfelt and Gatling guns. First World War combat experience greatly increased the importance of the machine gun. The United States Army issued four machine guns per regiment in 1912, but that allowance increased to 336 machine guns per regiment by 1919.
Heavy guns based on the Maxim such as the Vickers machine gun were joined by many other machine weapons, which mostly had their start in the early 20th century such as the Hotchkiss machine gun. Submachine guns (e.g., the German MP 18) as well as lighter machine guns (the Lewis gun, for example) saw their first major use in World War I, along with heavy use of large-caliber machine guns. The biggest single cause of casualties in World War I was actually artillery, but combined with wire entanglements, machine guns earned a fearsome reputation. The automatic mechanisms of machine guns were applied to handguns, giving rise to automatic pistols (and eventually machine pistols) such as the Borchardt (1890s) and later submachine guns (such as the Beretta 1918). Machine guns were mounted in aircraft for the first time in World War I. Firing "through" a moving propeller was initially evaded through metal reinforcement of the propeller, or simply avoiding the problem with wing-mounted guns or having a pusher propeller. By the late spring of 1915, the introduction of the gun synchronizer by the German Fliegertruppe made it possible to fire forward through a spinning propeller for fighter aircraft to come into their own, also requiring the ordnance used to have a closed bolt firing cycle for the best chance of success.
Interwar era and World War II
As better materials became available following the First World War, light machine guns became more readily portable; designs such as the Bren light machine gun replaced bulky predecessors like the Lewis gun in the squad support weapon role, while the modern division between medium machine guns like the M1919 Browning machine gun and heavy machine guns like the Browning M2 became clearer. Water jacket cooling systems were largely abandoned as unnecessary.
The interwar years also produced the first widely used and successful general-purpose machine gun, the German MG34. While this machine gun was equally able in the light and medium roles, it proved difficult to manufacture in quantity, and experts on industrial metalworking were called in to redesign the weapon for modern tooling, creating the MG42. This weapon was simpler, cheaper to produce, fired faster, and replaced the MG34 in every application except vehicle mounts, since the MG42's barrel changing system could not be operated when it was mounted.
Submachine guns evolved during the war, going from complex and finely made weapons like the Thompson submachine gun to weapons designed for mass-production and easy replacement like the Sten gun. On the Eastern Front, entire Soviet divisions were equipped with nothing but the PPSh-41 submachine gun. Experience in close-range city combat led to the German military desiring a weapon representing a compromise between the high fire volume of the SMG and the accuracy of a full-size rifle; after a false start with the FG 42, this led to the development of the MP44 select-fire assault rifle, the first weapon to be called such.
Experience with the MG42 led to the US issuing a requirement to replace the aging Browning Automatic Rifle with a similar weapon, which would also replace the M1919; simply using the MG42 itself was not possible, as the design brief required a weapon which could be fired from the hip or shoulder like the BAR. The resulting design, the M60 machine gun, was issued to troops during the Vietnam War.
As it became clear that a high-volume-of-fire weapon would be needed for fast-moving jet aircraft to reliably hit their opponents, Gatling's work with electrically powered weapons was recalled and the 20mm M61 Vulcan designed; a miniaturized 7.62mm version initially known as the "mini-Vulcan" and soon shorted to "minigun" was soon in production for use on helicopters, where the volume of fire could compensate for the instability of the helicopter as a firing platform.
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 caseless ammunition, anti-armor and anti-missile weapons.
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, or with a set of cartridges affixed to a plate that was inserted into the weapon). 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.
Modern machine guns are commonly 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 some medium machine guns. Another is a tripod, where the person holding it does not form a 'leg' of support. Medium and heavy machine guns usually use tripods. On ships, vehicles and aircraft machine guns are usually mounted on a pintle mount – basically a steel post that is connected to the frame or body. 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 example the US Helicopter Armament Subsystems.