Machine Guns
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 bullets per minute. Earlier machine guns were manually operated, for example, by turning a hand crank.
Sometimes machine gun is used loosely to mean any automatic firearm, for example in news media or legal code.
Machine Gun Overview
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 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 (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. Electrical 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 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 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, rather like the spread of a shotgun,
but continuous. 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 (selective fire).
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.
U.S. Marines and their M240G at Camp Hansen, Okinawa
Enlarge
U.S. Marines and their M240G at Camp Hansen, Okinawa
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.
History
Multi-shot guns have a long development, going as far back to the
1st century, with plans for a multi-shot arrow gun by Greek engineer
Hero of Alexandria. The Chinese had some 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. Leonardo Da Vinci devised plans for one in the 1400s,
and stretching back to some of the earliest firearms and attempts at
higher rates of fire, and some machine-gun-like traits happened as
early as the 1700s. However, it would not be until the mid-1800s
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 consider to be like machine guns, is a matter of debate
in many cases and can vary depending which language and exact
definition is used.
Early rapid-firing weapons
Among first known ancestor of multi-shot weapons 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 1 in. (25.4 mm)
caliber, flintlock revolver cannon able to fire 9 rounds before
reloading, intended for use on ships. According to Puckle, it was
able to fire round bullets at Christians and square bullets at
Turks. While ahead of its time, foreshadowing the designs of
revolvers, it was not adopted or produced.
In the early and mid-19th century, a number of rapid-firing weapons
appeared which offered multi-shot fire, and a number of
semi-automatic weapons as well as volley guns. Volley guns (such as
the Mitrailleuse) and double barreled pistols relied on duplicating
all parts of the gun. Pepperbox pistols did away with needing
multiple hammers but used multiple barrels. Revolvers further
reduced this to only needing a pre-prepared magazine using the same
barrel and ignitions. However, like the Puckle gun, they were still
only semiautomatic.
The coffee-mill gun of the Civil War featured both automatic loading
and single barrel, only separated functionally from the modern
machine gun by being hand-powered rather than using cartridges.
The Gatling gun, patented in 1861 by Richard Jordan Gatling, was the
first to offer controlled, sequential automatic fire with automatic
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
and was subsequently improved. Many were sold to other armies in the
late 1800s and continued to be used into the early 1900s, until they
were gradually supplanted by Maxim guns. The Gatlings were the first
widely used rapid-fire guns and, due to their multiple barrels,
could offer more sustained fire than the first generation of
air-cooled, recoil-operated machine guns. The weight, complexity,
and resulting cost of the multibarrel design meant recoil-operated
weapons, which could be made lighter and cheaper, would supplant
them. It would be another 50 years before the concept was again used
to allow extremely high rates of fire, such as in miniguns, and
automatic aircraft cannons.
Maxim gun
The first true machine gun was invented in 1883 by Hiram Maxim. The
"Maxim gun" used the recoil energy 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. Maxim's other great
innovation was 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 less crew, was lighter, and more useable
than earlier Gatling guns.
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. Submachine guns (e.g., the Thompson, or
"Tommy gun") as well as lighter machine guns (the BAR, for example)
saw their first major use in WW1, along with heavy use of
large-caliber machine guns. The biggest single cause of casualties
in WW1 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 solved in a variety of
ways, including the interrupter gear, metal reinforcement of the
propeller, or simply avoiding the problem with wing-mounted guns or
having a pusher propeller.
Interwar era and World War 2
During the interwar years, many new designs were developed, such as
the Browning M2 .50 caliber (12.7 mm) in 1933, which, along with
others, were used in World War II. The trend toward automatic
rifles, lighter machine guns, and more powerful submachine guns
resulted in a wide variety of firearms that combined characteristics
of an ordinary rifle and a machine guns. The Cei-Rigotti (1900s),
Fedorov Avtomat (1910s), AVS-36 Simonov (1930s), MP44, M2 Carbine,
AK-47, and AR-15 have come to be known as assault rifles (after the
German term sturmgewehr). Many aircraft were equipped with machine
cannons, and similar cannon (nicknamed "Pom-pom guns") were used as
antiaircraft weapons. The designs of Bofors of Sweden were widely
used by both sides and have greatly influenced similar weapons
developed since then. 1934 Germany developed during the interwar
years the first widley used and successful general-purpose machine
gun, the Maschinengewehr 34, which inspired many modern machine gun
developments. The successor of the Maschinengewehr 42, the MG3, is
still today in use in the German army.
Modern era
The Cold War era saw mostly a refinement of weapon types in the form
of lower weight and higher reliability. The semi-automatic rifles of
World War II vintage were almost totally replaced by lighter assault
rifles such as the M16 and Soviet AK-47. Infantry adopted
general-purpose machine guns like the American M60 for squad use,
using air cooling for lighter weight. Heavy machine guns were
retained for ground vehicles and fortifications. For aircraft use,
even heavy machine guns proved to lack killing power in the
air-to-air role, and by the late 1950s fighter aircraft armament had
almost totally switched to automatic cannons. Machine guns, with
lower recoil, remained popular for helicopters and for ground attack
aircraft, supplemented by new Gatling-style, electric multibarrel
weapons like the American Minigun. In police, special operations,
and other paramilitary roles, smaller automatic weapons, including
light submachine guns and machine pistols, proliferated, many
relying on ubiquitous pistol rounds.
The last major use of a manual machine gun, was a manual grenade
machine gun during the 1970s used on river boats in the Vietnam
Conflict. The manual type, the Mk 18 Mod 0 was replaced by fully
automatic ones such as the Mk 19 Mod 0.
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 has complex
machinery, weighs 85 pounds, and has very little penetration power
to show for it.
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. Lighter
machine guns, such as light and medium machine guns often have a
butt stock attached, while mounted 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 just iron sights.
Loading systems in early manual machine guns were often from a
hopper of loose (un-linked) cartridges. Manual 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.
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 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 way is disconnected from humans, as part of an armament
system, such as a tank coaxial or part of aircraft. These are
usually electrically fired and have complex sighting systems. (For
examples see US Helicopter Armament Subsystems).