Much like in Toms case, most investigations start with a crime having been committed. Forensic ballistics and firearm investigation start when there are bullets, cartridges, a weapon, or any combination of the above found at a crime scene. With the evidence, a crime lab can search for clues on these items that could lead to a suspect or possibly prove that the items were used in the crime. By comparing the markings on bullets or cartridges found at the scene with those fired from a suspect's weapon, a ballistics expert can often determine if the rounds came from the same weapon. Just the act of cycling a cartridge through a weapon without firing it can leave permanent scratches in the case that are unique to the weapon.
When a suspect's weapon is examined in the lab, it will be test fired into a box filled with cotton or a tank of water to provide the examiner with the bullets and cartridges with a known history. Using a microscope, the known cartridges are compared with the ones in question. With some patience, skill, and a little luck, experts can definitively say that a certain firearm and no other fired this bullet, or ejected this cartridge.
The value of luck cannot be overstated. A bullet may leave the muzzle of a weapon at over a thousand feet per second and slam into a concrete wall. This may deform the round beyond all recognition. Just about anything can and does happen to flying projectiles. Soft lead bullets are known to flatten out when they hit hard objects or even when they just hit plain water. Copper jacketed bullets may come loose from their jackets or disintegrate into numerous sharp pieces.
When a cartridge is fired, the pressure generated, which may be anything from two to over twenty tons per square inch, forces the case back against the breech face. This causes the individuality of the breech face to be imprinted on the base of the cartridge. In other words, the firearm leaves it's "finger-print" or "thumb-mark" is imprinted naturally varies with the pressure, but its image will always be present even though it is sometimes difficult to detect.
The bullet travels down the length of a gun barrel, and the hardened steel of the barrel wears the softer lead or copper of the bullet away ever so slightly. Tiny imperfections, which were introduced into the rifling, the gentle twisting groove pattern scribed into a gun barrel, when it was first manufactured, are created in the surface of the bullet. These markings are also unique as a fingerprint for the weapon. Guns of the same make produce striations, the scratch-like impressions left on the projectile, which are similar but not identical. There are usually several places where the hard steel of the weapon leaves its mark on the softer metal of the ammunition. The bullet is usually made of a lead or jacketed copper, both of which are softer than steel. The cartridge that remains in the gun will also have scratches and impressions made by the firing pin and extractor.
When a bullet or a casing is collected from a crime scene, examiners can often assist investigators by observing details about it without waiting for a firearm to be discovered. With a few...