Machine gun
A machine gun is a fully automatic and rifled firearm designed for sustained direct fire. Automatic firearms of caliber or more are classified as autocannons rather than machine guns.
As a class of military kinetic projectile weapons, machine guns are designed to be mainly used as infantry support weapons and generally used when attached to a bipod or tripod, a fixed mount or a heavy weapons platform for stability against recoil. Many machine guns also use belt feeding and open bolt operation, features not normally found on other infantry firearms.
Machine guns can be further categorized as light machine guns, medium machine guns, heavy machine guns, general-purpose machine guns, and squad automatic weapons. Submachine guns are not classified as machine guns as they fire handgun rather than rifle cartridges.
Modern overview
Unlike semi-automatic firearms, which require one trigger pull per round fired, a machine gun is designed to continue firing for as long as the trigger is held down. Nowadays, the term is restricted to relatively heavy crew-served weapons, able to provide continuous or frequent bursts of automatic fire for as long as ammunition feeding is replete. Machine guns are used against infantry, low-flying aircraft, small boats and lightly/unarmored land vehicles, and can provide suppressive fire or enforce area denial over a sector of land with grazing fire. 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 the 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, the great majority of designs 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 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. The term heavy machine gun originated in World War I to describe heavyweight medium machine guns, and persisted into World War II with Japanese Hotchkiss M1914 clones; today, however, it is used to refer to automatic weapons with a caliber of at least, but less than. A general-purpose machine gun is usually a lightweight medium machine gun that 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.
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 rounds and tracer ammunition 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 BMG 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 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, whether the cartridge is fired from a closed bolt or an open bolt, and whether the action used is locked or is some form of blowback.
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 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, sometimes with both of the latter presents.
Operation
Many machine guns are of the locked breech type, and follow this cycle:- Pulling the bolt assembly/bolt carrier rearward by way of the cocking lever to the point bolt carrier engages a sear and stays at rear position until trigger is activated making bolt carrier move forward
- Loading fresh round into chamber and locking bolt
- Firing round by way of a firing pin or striker hitting the primer that ignites the powder when bolt reaches locked position.
- Unlocking and removing the spent case from the chamber and ejecting it out of the weapon as bolt is moving rearward
- Loading the next round into the firing chamber. Usually, the recoil spring tension pushes bolt back into battery and a cam strips the new round from a feeding device, belt or box.
- Cycle is repeated as long as the trigger is activated by operator. Releasing the trigger resets the trigger mechanism by engaging a sear so the weapon stops firing with bolt carrier fully at the rear.
Design
Most modern machine guns are of the locking type, and of these, most utilize the principle of gas-operated reloading, which taps off some of the propellant gas from the fired cartridge, using its mechanical pressure to unlock the bolt and cycle the action. The first of these was invented by the French brothers Claire, who patented a gas operated rifle, which included a gas cylinder, in 1892. The Russian PK machine gun is a more modern example. Another efficient and widely used format is the recoil actuated type, which uses the gun's recoil energy for the same purpose. Machine guns, such as the M2 Browning and MG42, are of this second kind. A cam, lever or actuator absorbs part of the energy of the recoil to operate the gun mechanism.An externally actuated weapon uses an external power source, such as an electric motor or hand crank, to move its mechanism through the firing sequence. Modern weapons of this type are often referred to as Gatling guns, after the original inventor. They have several barrels each with an associated chamber and 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 and its relative immunity to overheating allow for a very 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 designs are intrinsically comparatively bulky and expensive and are therefore generally used with large rounds, 20 mm in diameter or more, often referred to as rotary cannon – though the rifle-calibre Minigun is an exception to this. Whereas such weapons are highly reliable and formidably effective, one drawback is that the weight and size of the power source and driving mechanism makes them usually 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. In contrast to the rotary format, such weapons have a single barrel and a recoil-operated carriage holding 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. A chain gun is a specific, patented type of revolver cannon, the name, in this case, deriving from its driving mechanism.
As noted above, 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 cooking off. To guard against cook-offs occurring, some kind of cooling system or design element is required. Early machine guns were often water-cooled and while this technology was very effective, the water jackets also added considerable weight to an already bulky design; they were also vulnerable to the enemies' bullets themselves. Armour could be provided, and in World War I, the Germans in particular often did this; but this added yet more weight to the guns. Air-cooled machine guns often feature quick-change barrels, passive cooling fins, or in some designs forced-air cooling, such as that employed by the Lewis Gun. Advances in metallurgy and the use of special composites in barrel liners have allowed for greater heat absorption and dissipation during firing. 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. Motorized Gatling guns can achieve the fastest firing rates of all, partly because this format involves extra energy being injected into the system from outside, instead of depending on energy derived from the propellant contained within the cartridges, partly because the next round can be inserted simultaneously with or before the ejection of the previous cartridge case, and partly because this design intrinsically deals with the unwanted heat very efficiently – effectively quick-changing the barrel and chamber after every shot. The multiple guns that comprise a Gatling being a much larger bulk of metal than other, single-barreled guns, they are thus much slower to rise in temperature for a given amount of heat, while at the same time they are also much better at shedding the excess, as the extra barrels provide a larger surface area from which to dissipate the unwanted thermal energy. In addition to that, they are in the nature of the design spun at very high speed during rapid fire, which has the benefit of producing enhanced air-cooling as a side-effect.
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.