Ballistics, the science that deals with the motion of projectiles such as bullets, shells, rockets, and aerial bombs. It is important to police officers who investigate crimes involving shooting, to artillerymen and naval gunnery officers, and to engineers who design firearms, missiles, bombsights, and fire-control systems.

Ballistics has three branches. Interior ballistics deals with the behavior of a projectile within a gun barrel. Exterior ballistics is concerned with the motion of a projectile in flight. Terminal ballistics deals with the effect of the projectile on its target.

Interior Ballistics

A firearm such as a rifle, pistol, or artillery piece fires a projectile as a result of the burning of its propellant, which usually is smokeless powder. When the powder is ignited, large quantities of gases are produced. These gases force the projectile through the barrel in much the same way that children blow peas from a pea-shooter. If the gas pressure is too small, the bullet or shell will not reach its target. If the pressure is too great the gun may blow up. Also, if the pressure changes with each shot, the velocity of the projectile will change and accuracy will be poor. How to regulate this pressure is one of the most important problems in interior ballistics.

Almost all types of firearms (except for shotguns and rocket launchers) have spiral grooves on the inside of their barrels. These grooves are called rifling. The depth, diameter, and number of turns vary in different weapons and differ slightly with each separate weapon of the same type. When a pistol or rifle is fired, the bullet's metal fills into the grooves. Irregularities, some visible only through a microscope, leave markings on the bullet. These markings help ballistic experts determine which weapon fired a given bullet, since all bullets from the same gun have similar markings. This type of investigation is useful in police work.

Exterior Ballistics

When a bullet or artillery shell leaves a gun, it spins like a top because of the spiral rifling. This spinning motion gives the projectile stability of flight. Some rocket missiles are made to spin by metal fins or by small auxiliary rocket engines. While in the air, the projectile is subject to various forces, such as gravity, air resistance, wind, and drift" caused by the spin. The effect of these forces must be considered when aiming a weapon to achieve accurate shooting.

Terminal Ballistics

The penetration of tank armor by armor-piercing ammunition is an important concern of terminal ballistics. Another is the damage done to the target by the blast effect and flying fragments of a shell or bomb. With nuclear weapons there is also damage from intense heat and radiation. The study of terminal ballistics helps scientists to develop more effective weapons and to devise means of defense against enemy weapons.