With almost every crime committed, there is always a weapon to be found. Whether the weapon was legally purchased or not, the semi-automatic pistol has always been a common sighting in crime involving shootings.

Before understanding how ballistic evidence, first one must know how a semi-automatic pistol operates. Most semi-automatic pistols rely on a magazine to store unfired bullet rounds and supply the actual gun itself with ammunition. Usually, the first round is loaded into the chamber by pulling the slide mechanism of the gun (also known as racking). When the gun is fired, during the recoil operation, the slide goes back; ejecting the expended round and automatically loads another unfired round.

There are two types of recoil operations: blowback and locked-breech (there is a variation of blowback and locked-breach mechanisms as well). In a blowback semi-automatic, the barrel is fixed into the frame and the breechblock is held against the barrel by the force of a spring. When fired, the slide goes back and the empty brass ejected from the right side of the gun. The spring in the breechblock slows down the rearward motion of the slide and beings to slide forward. At this point, an unfired round from the magazine is loaded into the chamber and the hammer, striker, and firing pin is reset to fire position.

In locked-breech, the barrel is temporarily locked into breechblock both recoil together at a small distance. During the recoil operation, the barrel temporarily shifts positions in order to eject and load a new round. More often is the blowback method is used because it is cheaper, however only with smaller caliber rounds. Lock-breached method is used for large rounds to insure that the rounds are not unintentionally ignited.

In the field of forensic science, forensic ballistics is the analyzing of firearm usage. In the early 18th century, the rifling in the barrel of a gun was used in order to increase accuracy and range of the round – and gave each gun it’s own distinct signature. These riflings were different in every gun, allowing ballistics to individualize each and every gun that has ever been used in a crime.