Machine Gun

General Overview: Calibers

There are two main different definitions of the upper limit of caliber for machine guns —

larger than 12.7 mm (.50 caliber) and larger than 20 mm — at which point they are generally

referred to as autocannons. In-between, there are weapons that have been called by either

name depending other traits; for instance, there have been weapons of roughly 15 mm that

were variably referred to as autocannons and machine guns.

Another factor is whether the weapon fires conventional rounds or explosive rounds.

Automatic weapons 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 caliber to barrel length (a long barrel for a small caliber).

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 — a manually-powered

rotary machine 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 all nearly all other semi and fully-automatic

firearms of 20th century.

The two major operation systems of modern automatic machine guns are gas operation (which

uses the gas generated from the burning powder to cycle the action), or recoil operation

(which uses the recoil generated from the ejecting bullet to cycle the action). The first

gas-operated machine gun was the Colt-Browning M1895. Another (minor) type is the

externally-powered machine gun. Rather than human manual power or bullet energy, a third

source (such as an electric motor) is used; these types are now called by more specific

names (see Minigun, Chaingun). The most common type of modern machine gun remains the

automatic, recoil-operated and belt-fed type. Eletrical and Gatling-type machine guns are

common on fighting aircraft and other vehicles.


Overview of modern automatic machine guns

Unlike semi-automatic firearms, which require one 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 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 sometimes 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 guns 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.

The machine gun's primary role in 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. This either halts an enemy attack or allows friendly forces to attack enemy

positions with less risk.

To this end, most light machine guns and general purpose machine guns are not designed for

high accuracy, as would be expected of a rifle. Most are designed with a small degree of

inaccuracy, referred to as the "cone of fire", because the rounds spread out as they travel

towards the target area. 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.

Assault rifles are a compromise between the pistol-caliber submachine gun and a traditional

rifle firing a full-power cartridge, allowing single-shot, burst and full-automatic fire

options.

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-material sniper rifles, such as the Barrett M82.


Components

All machine guns require the following components:

1. A feed system to load the firing chamber. Cartridges can be fed into the chamber by a

variety of methods, the most common being spring-fed magazines or ammunition belts.
2. A trigger mechanism to fire the round. This includes the actual trigger, a trigger

sear to catch the bolt, a bolt and a firing pin, as well as other components. Typically, the

act of pulling the trigger causes something to strike the primer on the round in the chamber

and disengages the sears. This allows continual cycling of the bolt until the trigger is

released. A sear then grabs the bolt or firing pin. This stops the machine gun at some point

in its cycle.
3. An extractor system to eject the spent or misfired cartridge. Usually this is fairly

simple. A pin on the side of the bolt catches a ridge on the cartridge and flicks it out an

ejection port.

These components form a mechanism which must be powered by something. If powered by a spring

absorbing the recoil of a fired cartridge, it is called recoil operated. If powered by the

expanding gases of a fired cartridge, it is called gas actuated. If it powered by an

external force, such as a motor, it is usually called a chain gun.


Operation
All machine guns follow a cycle:

* Removing the spent cartridge through an ejection port.
* Cocking the trigger mechanism so the weapon can be fired again.
* Loading the next round into the firing chamber. Usually spring tension or a cam forces

the new round and bolt back into the firing chamber.

A mechanism makes the firing pin fire the cartridge, activating the ejection and reloading

steps. The cycle repeats. This full cycle takes a fraction of a second and can thus occur

many times per second. The operation is basically the same, regardless of the means of

activating these mechanisms. Some examples:

* Machine pistols and submachine guns (like the World War II "grease gun," MAC-10 or the

Uzi) are usually blowback operated.

* Most assault rifles and squad automatic weapons are gas actuated. Some weapons, such

as the AR-15/M16, integrate the piston with the bolt. Others, such as the M15 and AK

patterns, attach the piston to a bolt carrier that unlocks and operates the bolt.

* A recoil-actuated machine gun uses the recoil to first unlock and then operate the

action. Heavy machine guns, such as the M2 .50 and Browning .50, are of this type. These can

be recognized by a large cocking lever needed to feed the first round.

* An externally actuated machine gun 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 chain guns in reference to their driving mechanism. Gatling

guns and revolver cannon have several barrels or chambers on a rotating carousel and a

system of cams that load, cock, and fire each mechanism progressively as it rotates through

the sequence. The continuous nature of the rotary action allows for an incredibly high

cyclic rate of fire, often several thousand rounds per minute. Not all chain guns use

multiple barrels or chambers, though. Chain 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. This is not possible if the force needed to eject the round comes from the

round itself. Chain guns are generally used with large shells, 20 mm in diameter or more,

though some, such as the M134 Minigun, fire smaller cartridges. They offer 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.

Heavy machine guns are often water cooled or have interchangeable barrels, which must be

changed periodically to avoid overheating. 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.

Not all machine guns strike the primer in the same way. In blowback machine guns, the act of

seating the round also fires the round. In gas operated and recoil-operated guns, a separate

step in the firing sequence is needed to strike the round. In a progressive-fire gun, the

firing pin is cycled by cams. In some automatic cannon, the primer is fired electrically.

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. This is

especially important in weapons like the 40 mm grenade launcher, where high explosives are

present in the rounds being fired.

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 weapons have a "safety" sear, which simply keeps the trigger from engaging.