subject: Interior Ballistics [print this page] Interior ballistics is the study of what happens inside a gun when it is fired. A gun, like a revolver or rifle, is loaded by placing a cartridge containing an explosive in the gun barrel. (In all but the heaviest artillery, the cartridge or shell itself contains the explosive. See the article On ammunition.) When the trigger is pulled, the explosive (usually gun powder) is set on fire. The gases that are formed push the bullet out of the gun with great force. This force can be measured with a tool called a crusher gauge.
The gauge is placed over a hole in the side of the gun barrel. When the gun is fired, the gases press against the side of the barrel and push down a cylinder of copper or lead in the pressure gauge. Ballistics experts can tell how much gas pressure there is in the barrel by measuring how far down the cylinder is pressed. The pressure behind a bullet forces it out of the muzzle, or end of the barrel, with great speed. Some bullets leave a gun at 2,000 miles an hour. The speed with which a bullet leaves 'a gun is called the muzzle velocity.
The muzzle velocity can be measured with a block of wood hanging from a string. This is called a ballistic pendulum. The bullet is fired into the wood block. By measuring how far the block swings back, and also weighing the block and the bullet, ballistics experts can find out the muzzle velocity. With such information, ballistics experts can tell how much powder to put in a cartridge, how heavy the bullet should be, and how wide and long the gun should be for it to fire accurately.
