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How Machine Guns Work
by Tom Harris

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 arsenal.

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 gunpowder.

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 How Flintlock Guns Work.

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 weapons.)

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 revolver.

Click on the trigger to see how a revolver fires.

This gun has a revolving cylinder, with six breeches for six cartridges. When you pull the trigger on a revolver, several things happen:

  1. Initially, the trigger lever pushes the hammer backward.
    • 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 barrel.
  2. When you pull the trigger all the way back, the lever releases the hammer.
  3. The compressed spring drives the hammer forward.
  4. The hammer slams into the primer at the back of the cartridge, igniting the primer.
  5. The primer sets off the propellent.
  6. 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 misfire.

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 below.

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 barrels.

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 evolution.

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 forever.

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 this power:

  • Recoil systems
  • Blowback systems
  • Gas mechanisms

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 backward.

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 diagram below.

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 work.

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 '40s.

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 the "Feeding and Size" section to find out how these components work.

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 ground combat scenarios.

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 mechanism

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 (6).

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 ejection port.

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 re-use.

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 helicopters.

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.

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