Bullets and missiles

Last updated: May 12, 2008.

Has any other single invention changed history quite so much as explosives? As the power behind bombs and missiles, chemical explosives have made possible most of the great wars of the last 1000 years or so, altering the course of history time and time again. Before the invention of gunpowder, the first crude chemical explosive, people had to fight their enemies hand-to-hand on the battlefield with crude weapons like swords and spears. Today, you don't even have to be able to see your enemy—let alone touch him: it's easy to drop bombs from airplanes, shoot them from submarines, or launch them on rockets from one side of the Earth to the other. But even though modern missiles are incredibly sophisticated, the basic science and technology behind them is pretty much the same as it was 1000 years ago!

Photo: Oil painting of a sailor loading ammunition, 1942, by McClelland Barclay (1891-1943), who was killed onboard ship by a Japanese torpedo during his service as a US war artist. Picture courtesy of the US Department of the Navy/Navy Historical Centre.

How guns fire bullets

Bullets and missiles come in all shapes and sizes. At 21.8 meters (71 ft) long, one of the world's biggest intercontinental ballistic missiles, the US Airforce LGM-118A Peacekeeper, is three times the length of a station wagon! But it works pretty much the same way as a handgun bullet the size of your pinkie.

Photo: Launch of a Peacekeeper missile. By Don Sutherland, courtesy of the US Defense Visual Information Center Centre.

Bullets are a bit like fireworks and they are arranged in three sections: the primer, the propellant, and the bullet proper. At the back, the primer (or percussion cap) is like the fuse of a firework: a small fire that starts a bigger one. The next section of the bullet, effectively its "main engine," is a chemical explosive called a propellant. Its job is to power the bullet through the air from the gun to the target. The front part of the bullet is a tapering metal cylinder that hits the target at high speed. It tapers to a point to help it penetrate through metal, flesh, or whatever else the target may be made from.

Bullets are designed to be (relatively) safe until the moment when you fire them. When you pull the trigger of a gun, a spring mechanism hammers a metal firing pin into the back end of the bullet, igniting the small explosive charge in the primer. The primer then ignites the propellant—the main explosive that occupies about two thirds of a typical bullet's volume. As the propellant chemicals burn, they generate lots of gas very quickly. The gas shoots from the back of the bullet, increasing the pressure behind it, and forcing it down the gun barrel at extremely high speed (300 m/s or 1000 ft/s is typical in a handgun).

The propellant chemicals in a handgun bullet are not designed to explode suddenly, all at once: that would blow the whole gun open and very likely kill the person firing it. Instead, they are supposed to start burning relatively slowly, so the bullet moves off smoothly down the gun. They burn faster as the bullet accelerates down the barrel, giving it a maximum "kicking" force just as it comes out of the end. As the bullet emerges, the whole gun recoils (leaps backward) because of a basic law of physics called "action and reaction" (or Newton's third law). When the gas from the explosion shoots the bullet forwards with force, the whole gun jolts backwards with an equal force in the opposite direction.

Photo: Unlike a conventional weapon, this 75mm recoilless rifle doesn't jerk back when fired. It's open at the back so the explosive blast escapes from the rear of the gun, eliminating the usual recoil. You can clearly see the heat of the explosive charge exploding from the front and the blast simultaneously shooting out from the rear. The gunner barely moves at all. By Blake R. Waltman, courtesy of the US Army.

The explosion that fires a bullet happens in the confined space of the gun barrel. As the bullet flies out of the gun, the pressure of the explosion is suddenly released. That's what makes a gun go BANG! It's a bit like uncorking a bottle of wine at much higher speed and pressure. Some bullets also make noise because they go so quickly. The fastest bullets travel at around 3000 km/h (over 1800 mph) —about three times the speed of sound. Like a supersonic (faster-than-sound) jet fighter, these bullets make shock waves as they roar through the air.

How bullets travel

Gun barrels have spiralling grooves cut into them that make bullets spin around very fast as they emerge. A spinning bullet is like a gyroscope: a sort of "stubborn" spinning wheel that always tries to keep turning the same way. If you try to tilt a gyroscope while it's spinning, it will try to resist whatever force you apply and, if you let go, it will soon tilt back the other way. This is why, when things are spinning, they are very hard to deflect from their path. We call this idea gyroscopic inertia or stability. A bullet behaves in exactly the same way: once it's spinning, it follows a straighter path as it goes through the air, so it's harder to deflect and much more likely to reach its target.

Photo: US army gunner with a .50 caliber machine gun in Iraq. Picture by Sgt. Matthew Acosta, courtesy of the US Army.

We think of bullets flying in perfectly straight lines—but nothing could be further from the truth. Several different forces act on a bullet as it goes through the air. Over very short distances, bullets do follow more or less a straight line. Over longer distances, they follow a slight downward curve because gravity tugs them toward the ground as they go along. Air resistance and the spinning, gyroscopic motion of a bullet complicate things too. Usually, because of recoil, the person firing wobbles the gun slightly when the bullet emerges. When all these factors—the bullet's motion, gravity, air resistance, recoil, and spinning—add together, they make a bullet follow a very complicated corkscrew path as it flies through the air.

Why bullets do damage

When things move, they have momentum. The faster they move and the heavier they are, the more momentum they have. A truck trundling along slowly has a lot of momentum because it weighs so much. Even though bullets are tiny, they have lots of momentum because they go so fast. They also have huge amounts of kinetic energy, which they get from the chemical energy of the burning propellant. Bullets do damage when they transfer their energy to the things they hit. The faster something loses its momentum, the more force it produces. A rifle bullet coming to a stop in a tenth of a second produces as much force as a heavy, slow moving truck coming to rest in 10 seconds. Imagine being hit by a truck—and you'll have some idea why bullets do so much damage.

Photo: Left: Close-up of a bullet hole in the fuselage of a US Air Force plane, fired on while delivering relief supplies in Somalia. Public domain picture courtesy of the US Department of Defense.

Photo: Right: Now that's what I call kinetic energy! This is what happens when you fire a 7g (0.25 oz) projectile at a velocity of 25,000 km/h (16,000 mph) into a cast aluminum block. This huge hole has been made by something weighing about as much as an iron nail! Even if something is that tiny, if it's travelling at very high speed it will have enough kinetic energy to do a lot of damage. Photo by R.D. Ward courtesy of the US Defense Visual Information Center (DVIC).

How to protect yourself

There's no completely sure way of protecting yourself against the energy of a speeding bullet, though protective materials certainly help. You can find out more in our article about bulletproof glass.