Bullets leave behind fingerprints, too

Forensic scientist Anna Bradley, from the University of Western Australia (UWA), is undertaking the world’s largest bullet lead study, building on research the FBI started when US President John F Kennedy was assassinated in 1963. Bradley is looking to track the unique ‘fingerprint’ of bullets in the hope of solving crimes.

“Around 20% of homicides and armed robberies in Australia involve the use of a gun,” Bradley said. “But if the firearm is not recovered or the bullet is fragmented, this can make things tricky for the physical examination. If a bullet from a crime scene can be ‘fingerprinted’, which means determining its elemental composition, then it can be compared to the composition of ammunition found in the suspect’s car or house or in a recovered firearm.”

As part of her PhD research, Bradley enlisted the help of two Australian ammunition manufacturers - one big, one small; each with different ways of making bullets - to test her hypothesis and found that the elemental signature of bullets remains unchanged throughout the manufacturing process. She also collaborated with the Western Australia Police Service, who provided reference ammunition to build up a database of different bullets.

After shooting slaughtered pigs’ heads with different ammunition, X-raying the skulls and extracting the lead shot and bullet fragments, Bradley was able to match the extracted samples to their unique production batch with 97% accuracy. By being able to determine up to 19 trace elements found in bullet lead - including arsenic, gold and mercury - she could trace a bullet back to its point of origin, no matter where it was manufactured or where the lead was sourced.

The FBI dropped the practice of compositional analysis in 2005, having only managed to determine seven trace elements in bullet lead. Bradley’s improvements may see the method resurrected.

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