Physics of Firearms

A firearm, in the most basic form, consists of a barrel, an action, a firing mechanism, a stock and a sighting device. No matter the form of the firearm, the same basic principles of physics and chemistry apply. The study of these principles is called ballistics. There are three types of ballistics, internal, external, and terminal. Internal ballistics is the study of the flight of a projectile inside a firearm. External ballistics is the study of the flight of a bullet after leaving the muzzle and until it hits the target. Terminal ballistics is the study of the effects of a projectile on a given target. Because this essay deals with the flight of the bullet, terminal ballistics will not be discussed.

Internal ballistics is the study of the flight of a projectile inside the firearm. The main components of internal ballistics include the firing process, rifling, and recoil.

The Firing Process

The firing process begins when the trigger of a gun is pulled. This releases the firing pin, allowing it to collide with and ignite the primer and cartridge. As the cartridge burns, it produces a hot gas that increases the temperature and pressure inside the cartridge. Once the pressure of the cartridge exceeds the friction holding the bullet in place, the bullet moves down the barrel of the rifle. When the bullet leaves the barrel the pressure decreases greatly and the hot air expands as it leaves the barrel. This is the cause of the loud bang when a rifle is shot.

Rifling

The accuracy of a firearm depends greatly on the spin of the bullet as it leaves the rifle. The spin of a bullet is caused by the grooves (rifling) cut on the inside of the barrel of the gun. Without a spin on the bullet, there would be no gyroscopic forces ...

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.... Unless a shooter has a ballistic simulator, they rely on drop tables which are provided by cartridge manufacturers.

Shooting at an Angle

In the "zeroing a rifle" section we required that the line of sight to the target was horizontal. What if this is not the case? The picture below shows the path a typical bullet will take when shot at a target that is not at the same elevation as the shooter, or upward angle. If a shooter shoots at a target whos line of sight to the target is not horizontal and the line of sight distance (or slant range (Rs)) is equal to the zero range of the rifle and he aims directly at the target, the shooter will shoot over the target. (See diagram) To correct for this, the shooter must compensate for the zeroing of the rifle. This is done by finding the horizontal range to the target (Rh). One can use trigonometry: Rh = Rs cos(a).