Ballistic fingerprinting

Forensic firearm examination is the forensic process of examining the characteristics of firearms as well as any cartridges or bullets left behind at a crime scene. Specialists in this field are tasked with linking bullets and cartridges to weapons and weapons to individuals. Obliterated serial numbers can be raised and recorded in an attempt to find the registered owner of the weapon. Examiners can also look for fingerprints on the weapon and cartridges, and then viable prints can be processed through fingerprint databases for a potential match.

By examining unique striations, or markings, left behind on the bullet as it passes through the barrel and on the cartridge as it is hit by the firing pin, individual spent rounds can be linked back to a specific weapon. Known exemplars taken from a seized weapon can be directly compared to samples recovered from the scene using a comparison microscope. Striation images can also be uploaded to any existing national databases. Furthermore, these markings can be compared to other images in an attempt to link one weapon to multiple crime scenes. Like all forensic specialties, forensic firearm examiners are subject to being called to testify in court as expert witnesses.

The ability to compare ammunition is a direct result of the invention of rifling around the turn of the 16th century. By forcing the bullet to spin as it travels down the barrel of the weapon the bullet's accuracy greatly increases. At the same time, the rifling leaves marks on the bullet that are indicative of that particular barrel. Prior to mass production of firearms, each barrel and bullet mold was hand made by gunsmiths making them unique. The first successful documented case of forensic firearm examination occurred in 1835 when a member of the Bow Street Runners in London matched a recovered bullet from a murder victim to a specific mold in a suspect's home confirming that he made the bullet. Further evidence that the bullet maker was the perpetrator was found in his home and he was convicted. As manufacturing and automation replaced hand tools, the ability to compare bullets became impossible due to the standardization of molds within a specific company. However, experts in the field postulated that there were microscopic differences on each barrel left during the manufacturing process. These differences were a result of wear on the machines and since each new weapon caused a tiny amount of wear, each barrel would be slightly different from every other barrel produced by that company. Also, each bullet fired from a specific barrel would be printed with the same marks, allowing investigators to identify the weapon that fired a specific bullet.

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