Historians count the machine gun among the most important
technologies of the past 100 years. As much as any other
factor, it set the brutal, unrelenting tone of World War I and
World War II, as well as most of the wars since that time.
With this machine, one soldier could fire hundreds of bullets
every minute, mowing down an entire platoon in only a few
passes. Military forces had to develop heavy battle equipment,
such as tanks, just to withstand this sort of barrage. This
single weapon had a profound effect on the way we wage war.
Photo courtesy Department
of Defense U.S. Marines
fire a M-240G machine gun during training exercises at
Camp Lejeune Marine Corps Base in North Carolina. Medium
machine guns such as this one are an essential element
in the modern
In light of their monumental role in history, it's somewhat
surprising how simple machine guns really are. These weapons
are remarkable feats of precision engineering, but they work
on some very basic concepts. In this edition of HowStuffWorks,
we'll look at the standard mechanisms machine guns use to spit
out bullets at such a furious rate.
Ballistic Background To understand how
machine guns work, it helps to know something about firearms
in general. Almost any gun is based on one simple concept: You
apply explosive pressure behind a projectile to launch it down
a barrel. The earliest, and simplest, application of
this idea is the cannon.
A cannon is just a metal tube with a closed end and an open
end. The closed end has a small fuse hole. To load the cannon,
you pour in gunpowder (a mixture of charcoal, sulfur and
potassium nitrate), and then drop in a cannonball. The
gunpowder and cannonball sit in the breech, the rear
part of bore, which is the open space in the cannon. To
prepare the gun for a shot, you run a fuse (a length of
flammable material) through the hole, so it reaches down to
the gunpowder. To fire the cannon, you light the fuse. The
flame travels along the fuse, and finally reaches the
When you ignite gunpowder, it burns rapidly, producing a
lot of hot gas in the process. The hot gas applies much
greater pressure on the powder side of the cannonball than the
air in the atmosphere applies on the other side. This propels
the cannonball out of the gun at high speed.
The first handheld guns were essentially miniature cannons;
you loaded some gunpowder, a steel ball and lit a fuse.
Eventually, this technology gave way to trigger-activated
weapons, such as the flintlock
gun and the percussion cap.
A percussion cap gun (left) and a flintlock
gun (right), two important steps on the way to modern
firearms. To learn more about these weapons, check out
Flintlock guns ignited gun powder by producing a tiny
spark, while percussion caps used mercuric fulminate, an
explosive compound you could ignite with a sharp blow. To load
a percussion cap gun, you poured gunpowder into the breech,
stuffed the projectile in on top of it, and placed a mercuric
fulminate cap on top of a small nipple. To fire the gun, you
cocked a hammer all the way back, and pulled the gun's
trigger. The trigger released the hammer, which swung forward
onto the explosive cap. The cap ignited, shooting a small
flame down a tube to the gunpowder. The gunpowder exploded,
launching the projectile out of the barrel. (Check out How
Flintlock Guns Work for more information on these
The next major innovation in the history of firearms was
the bullet cartridge. Simply put, cartridges are a
combination of a projectile (the bullet), a propellant
(gunpowder, for example) and a primer (the explosive cap), all
contained in one metal package.
Needless to say, cartridges were a phenomenal success. In
fact, they form the basis for most modern firearms. In the
next section, we'll see how these sorts of weapons work.
You Say You Want a Revolution In the last
section, we saw that a cartridge consists of a primer,
a propellant and a projectile, all in one metal package. This
simple device is the foundation of most modern firearms. To
see how this works, let's look at a standard double-action
Click on the trigger to see how a revolver
This gun has a revolving cylinder, with six breeches for
six cartridges. When you pull the trigger on a revolver,
several things happen:
Initially, the trigger lever pushes the hammer
As it moves backward, the hammer compresses a metal
spring in the gun stock (the handle).
At the same time, the trigger rotates the cylinder so
the next breech chamber is positioned in front of the gun
When you pull the trigger all the way back, the lever
releases the hammer.
The compressed spring drives the hammer forward.
The hammer slams into the primer at the back of the
cartridge, igniting the primer.
The primer sets off the propellent.
The exploding propellent drives the bullet out of the
gun at high speed.
The inside of the barrel has a
spiral groove cut into it, which serves to spin the
bullet as it exits the gun. This gives the bullet better
stability as it flies through the air, increasing accuracy.
When the propellant explodes, the cartridge case expands.
The case temporarily seals the breech, so all the expanding
gas pushes forward rather than backward.
Revolvers, which come in a range of shapes
and sizes, are one of the most popular gun designs of
all time. Their design is so simple that they almost
never jam or
Obviously, this sort of gun is easier to use than a
flintlock or a percussion cap weapon. You can load six shots
at a time, and you only have to pull the trigger to fire. But
you're still fairly limited: You have to pull the trigger for
every shot, and you need to reload after six shots. You also
have to eject the empty shells from the cylinders manually.
In the 1800s, gun manufacturers worked up a number of
mechanisms to address these problems. A lot of these early
machine guns combined several barrels and firing hammers into
a single unit. Among the most popular designs was the
Gatling gun, named after its inventor Richard Jordan
Gatling. You can see how this weapon works in the diagram
This weapon, the first machine gun to gain widespread
popularity, consists of six to 10 gun barrels positioned in a
cylinder. Each barrel has its own breech and firing pin
system. To operate the gun, you turn a crank, which
revolves the barrels inside the cylinder. Each barrel passes
under an ammunition hopper, or carrousel
magazine, as it reaches the top of the cylinder. A new
cartridge falls into the breech, and the barrel is loaded.
Each firing pin has a small cam head that catches hold of a
slanted groove in the gun body. As each barrel revolves around
the cylinder, the groove pulls the pin backward, pushing in on
a tight spring. Just after a new cartridge is loaded into the
breech, the firing-pin cam slides out of the groove, and the
spring propels it forward. The pin hits the cartridge, firing
the bullet down the barrel. When each barrel revolves around
to the bottom of the cylinder, the spent cartridge shell falls
out of an ejection port.
Photo courtesy Department
of Defense A U.S. airman
fires a GAU-17 mini-gun from a UH-1 Huey during training
exercises in Australia. Mini-guns are modern updates of
the Gatling gun, with an electric motor, rather than a
hand-crank, to rotate the
The Gatling gun played an important role in several 19th
century battles, but it wasn't until the early 20th century
that the machine gun really established itself. In the next
section, we'll look at the next major step in machine gun
Fully Automatic The Gatling gun is often
considered a machine gun because it shoots a large number of
bullets in a short amount of time. But unlike modern machine
guns, it is not fully automatic. You have to keep cranking if
you want to keep shooting. The first fully automatic machine
gun is credited to an American named Hiram Maxim.
Maxim's remarkable gun could shoot more than 500 rounds per
minute, giving it the firepower of about 100 rifles.
Hiram Maxim and one of his early machine gun
designs: When Maxim introduced his weapon to the British
army in 1885, he changed the battlefield
The basic idea behind Maxim's gun, as well as the hundreds
of machine gun designs that followed, was to use the power of
the cartridge explosion to reload and re-cock the gun after
each shot. There are three basic mechanisms for harnessing
The first automatic machine guns had a recoil-based
system. In nature, every action has an equal and opposite
reaction. This principle is responsible for the recoil effect
in guns. When you propel a bullet down the barrel, the forward
force of the bullet has an opposite force that pushes the gun
In a gun built like a revolver, this recoil force just
pushes the gun back at the shooter. But in a recoil-based
machine gun, moving mechanisms inside the gun absorb
some of this recoil force. You can see how this works in the
Click and hold the trigger to see how a
recoil-action gun fires. For simplicity's sake, this animation
doesn't show the cartridge loading, extraction and ejection
mechanisms. See the "Feeding
and Size" section to find out how these components work.
Here's the process: To prepare this gun to fire, you pull
the breech bolt (1) back, so it pushes in the rear
spring (2). The trigger sear (3) catches onto the
bolt and holds it in place. The feed system runs an
ammunition belt through the gun, loading a cartridge into the
breech (more on this later). When you pull the trigger, it
releases the bolt, and the spring drives the bolt forward. The
bolt pushes the cartridge from the breech into the chamber.
The impact of the bolt firing pin on the cartridge
ignites the primer, which explodes the
propellant, which drives the bullet down the barrel.
The barrel and the bolt have a locking mechanism
that fastens them together on impact. In this gun, both the
bolt and the barrel can move freely in the gun housing. The force of the
moving bullet applies an opposite force on the barrel, pushing
it and the bolt backward. As the bolt and barrel slide
backward, they move past a metal piece that unlocks them. When
the pieces separate, the barrel spring (4) pushes the barrel
forward, while the bolt keeps moving backward.
The bolt is connected to an extractor, which removes
the spent shell from the barrel. There are a number of
extractor systems in modern guns, but the basic idea in all of
them is fairly simple. In a typical system, the extractor has
a small lip that grips onto a narrow rim at the base of the
shell. As the bolt recoils, the extractor slides with it,
pulling the empty shell backward.
The backward motion of the bolt also activates the
ejection system. The ejector's job is to remove the
spent shell from the extractor and drive it out of an ejection
port (more on this later).
When the spent shell is extracted, the feeding system can
load a new cartridge into the breech. If you keep the trigger
depressed, the rear spring will drive the bolt against the new
cartridge, starting the whole cycle over again. If you release
the trigger, the sear will catch hold of the bolt and keep it
from swinging forward.
In the next section, we'll look at the other main machine
gun mechanisms: the blowback and gas systems.
Blowback and Gas A blowback system is
something like a recoil system, except the barrel is fixed in
the gun housing and the barrel and bolt do not lock together.
You can see how this mechanism works in the diagram below.
Click and hold the trigger to see how a
blowback-action gun fires. For simplicity's sake, this
animation doesn't show the cartridge loading, extraction and
ejection mechanisms. See the "Feeding
and Size" section to find out how these components
This gun has a sliding bolt (3) held in place by a spring,
a spring-driven cartridge magazine (5), and a trigger
mechanism (1). When you slide the bolt back, the trigger sear
(2) holds it in place. When you pull the trigger, the sear
releases the bolt, and the spring drives it forward. After the
bolt chambers the cartridge, the firing pin sets off the
primer, which ignites the propellant.
The explosive gas from the cartridge drives the
bullet down the barrel. At the same time, the gas pressure
pushes in the opposite direction, forcing the bolt backward.
As in the recoil system, an extractor pulls the shell out of
the barrel, and the ejector forces it out of the gun. A new
cartridge lines up in front of the bolt just before the spring
pushes the bolt forward, starting the process all over again.
This continues as long as you hold the trigger down and there
is ammunition feeding into the system.
Photo courtesy NARA A U.S. Marine, fighting in Okinawa, Japan,
during World War II, fires a military-issue Thompson's
submachine gun. The Thompson's, commonly known as the
"Tommy gun," was a popular weapon with both soldiers and
gangsters in the 1930s and
The gas system is similar to the blowback system,
but it has some additional pieces. The main addition is a
narrow piston, attached to the bolt, that slides back
and forth in a cylinder positioned above the gun barrel. You
can see how this system works in the diagram below.
Click and hold the trigger to see how a gas-action
gun fires. For simplicity's sake, this animation doesn't show
the cartridge loading, extraction and ejection mechanisms. See
and Size" section to find out how these components
This gun is basically the same as a blowback-system gun,
but the rear force of the explosion doesn't propel the bolt
backward. Instead, the forward gas pressure pushes the
bolt back. When the bolt swings forward to fire a cartridge,
it locks onto the barrel. Once the bullet makes its way down
the barrel, the expanding gasses can bleed off into the
cylinder above the barrel. This gas pressure pushes the piston
backward, moving it along the bottom of the bolt. The sliding
piston first unlocks the bolt from the barrel, and then pushes
the bolt back so a new cartridge can enter the breech.
These diagrams only depict particular examples of how these
systems work. There are hundreds of machine gun models in
existence, each with its own specific firing mechanism. These
guns differ in a number of other ways as well. In the next
section, we'll look at some of the key differences between
various machine gun models.
Feeding and Size One of the main differences
between different machine gun models is the loading mechanism.
One popular system is the spring-operated magazine. In this
system, a spring pushes cartridges in a magazine casing up
into the breech. The main advantages of this mechanism are
that it is reliable, lightweight and easy to use. The main
disadvantage is that it can only hold a relatively small
amount of ammunition.
Photo courtesy Department
of Defense A U.S. Marine
training with an M16A2 5.56mm assault rifle: Assault
rifles, relatively lightweight, magazine-fed automatic
weapons, are the gun of choice for a wide range of
A similar system is the ammunition hopper, such as
the one used in a Gatling gun. Hoppers are just metal boxes
that fit on top of the machine gun mechanism. One by one, the
cartridges fall out of the hopper and into the breech. Hoppers
can hold a good amount of ammunition, and they're easy to
reload, but they are fairly cumbersome and only work if the
gun is positioned right side up.
For sheer volume of ammunition, the belt system is
usually the best option. Ammunition belts consist of a long
string of cartridges fastened together with pieces of canvas
or, more often, attached by small metal links. Guns that use
this sort of ammo have a feed mechanism driven by the recoil
motion of the bolt. You can see how this sort of mechanism
works in the diagram below.
Top-view diagram of a common feed
The bolt (1) in this gun has a small cam roller (5)
on top of it. As the bolt moves, the cam roller slides back
and forth in a long, grooved feed cam piece (2). When
the cam roller slides forward, it pushes the feed cam to the
right against a return spring (6). When the cam roller
slides backward, the spring pushes the cam back to the left.
As it moves, the feed cam pivots a feed cam lever from
side to side. The feed cam lever is attached to a
spring-loaded pawl (8), a curved gripper that rests on
top of the ammunition belt. As the cam and lever move, the
pawl moves out, grabs onto a cartridge and pulls the belt
through the gun. When the bolt moves forward, it pushes the
next cartridge into the chamber. You can see how this works in
the diagram below.
Click and hold the trigger to see how the
loading and ejection system works.
The feed system drives the ammunition belt through
cartridge guides (2) just above the breech. As the bolt
slides forward, the top of it pushes on the next cartridge in
line. This drives the cartridge out of the belt, against the
chambering ramp (3). The chambering ramp forces the
cartridge down in front of the bolt. The bolt has a small
extractor, which grips the base of the cartridge shell
when the cartridge slides into place. As the cartridge slides
in front of the bolt, it depresses the spring-loaded ejector
When the firing pin hits the primer, propelling the bullet
down the barrel, the explosive force drives the operating rod
and attached bolt backward. The extractor pulls the spent
shell out of the breech. As the bolt keeps moving backward,
the spring-loaded ejector pushes on the base of the shell.
When the shell clears the chamber wall, the ejector springs
forward, popping the shell out of the gun through the
This system lets you fire continuously without
reloading. Theoretically, you could make ammunition belts of
any length, so they are a great means of providing a constant
supply of ammunition. The problem is that the belt is fairly
cumbersome, and there's a relatively high likelihood of the
feed mechanism jamming.
The Vickers MK1 belt-fed machine gun, a
favorite of the British military, played a crucial role
in World War I and World War II. The gun is cooled with
a special water-filled jacket. As the water boils, the
steam flows out to a collection can, where it condenses
back into a liquid for
Heavy belt-fed machine guns, usually mounted on a tripod or
a vehicle, may need more than one operator. Individual troops
usually carry light weapons, with extendible bipods or tripods
for stability. Smaller automatic guns that use cartridge
magazines are classified as automatic rifles,
assault rifles or submachine guns. In a general
sense, the term "machine gun" describes all automatic weapons,
including these smaller weapons, but it also used to describe
heavy belt-fed guns specifically.
Photo courtesy Department
of Defense Heavier
machine guns, such as this .50-caliber M-2, may be
mounted on tanks, jeeps, boats and
Gun manufacturers are continually adding new modifications
to machine guns, but the basic mechanism has remained the same
for more than a hundred years. Whether or not you've ever held
a machine gun, or even seen one, this device has had a
profound effect on your life. Machine guns have had a hand in
dissolving nations, repressing revolutions, overthrowing
governments and ending wars. In no uncertain terms, the
machine gun is one of the most important military developments
in the history of man.