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 Air Force LGM-118A Peacekeeper, is three times the length of a station wagon (estate car)! But it works pretty much the same way as a handgun bullet the size of your pinkie.

What’s inside a bullet cartridge?

When people talk about a “bullet” in everyday language, they often mean a cartridge, which is a three-part vehicle with the actual bullet mounted on the very end. The cartridge is the thing you load into a rifle; the bullet is the part of a cartridge that fires out the end. Cartridges 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 small fire that starts a bigger one. The next section of the cartridge, effectively the bullet’s “main engine,” is a chemical explosive called a propellant. Its job is to power the bullet down the gun and through the air to the target. The front part of the cartridge is the actual bullet: a tapering metal cylinder that hits the target at high speed. It tapers to a point mainly to reduce air resistance, so it goes faster and further, but also to help it penetrate metal, flesh, or whatever else the target may be made from (it must penetrate the target before it can do damage).

What happens when you fire?

Bullet cartridges 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 cartridge, igniting the small explosive charge in the primer. The primer than ignites the propellant—the main explosive that occupies about two-thirds of a typical cartridge’s volume. As the propellant chemicals burn, they generate lots of gas very quickly. The sudden, high pressure of the gas splits the bullet from the end of the cartridge, forcing it down the gun barrel at extremely high speed (300 m/s or 1000 ft/s is typical in a handgun). It’s only the bullet that fires from the gun; the rest of the cartridge stays where it is. It has to be ejected after firing (sometimes manually, sometimes automatically) to make way for the next cartridge—and the next shot.

Why bullets do damage

A moving object has momentum, which is the product of its mass and its velocity. The faster something moves and the heavier it is, the more momentum it has. 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. And because they go fast, they also have huge amounts of kinetic energy, which they get from the chemical energy of the burning propellant. (Remember that kinetic energy is related to the square of an object’s velocity—so if it goes twice as fast, it has four times the energy.)

Bullets do damage when they transfer their energy to the things they hit. The faster something loses its momentum, the more force it produces. (One way to define force is as the rate at which an object’s momentum changes.) 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!

More energy = more damage?

It’s easy to conclude from this that a bullet needs to have as much energy as possible to do the maximum amount of damage but, unfortunately, it’s not quite that simple. A rifle bullet has many times the velocity and kinetic energy of a handgun bullet, so much so that it will typically enter one side of a target, whiz straight through, and fly out the other side. If a bullet leaves the target at high speed, it’s taking valuable energy with it. So what we really want from a bullet is that it deposits as much energy as possible inside the target, either stopping entirely without exiting or leaving with the minimum possible velocity. There are various ways to achieve this.

The crudest way is for a bullet to expand as it enters the target. A bullet that expands has a bigger cross-sectional area, so it creates a bigger hole (or wound) in the target. It takes more energy to make a bigger hole in something: we need to use more force over the same distance, so we say the bullet “does more work” and uses more energy in the process. Bullets can be designed to expand by making them hollow at the pointed end and, after impact, they expand and squash down into a shape that looks like a button mushroom; that’s why deforming bullets are called hollow-point or mushrooming bullets (Dum-Dum bullets is another common name for them, taken from the place in India where they were invented in the late 19th century). International law has restricted the use of expanding bullets like this in wartime since 1899, but some police forces do still use them. That’s partly because expanding bullets do so much damage that they immediately incapacitate their target, but also because a mushrooming bullet, fired in self-defense (maybe in a crowded city street), is much more likely to stay inside its target and less likely to injure an innocent bystander by accident. Soft-point bullets work similarly, only using a soft lead tip instead of a hollow point, but expand more slowly and typically penetrate deeper.