Australasian Science: Australia's authority on science since 1938

Bullets Fingerprinted

Trace elements in lead can now be used to trace help forensic scientists to “fingerprint” bullets.

Anna Bradley of the University of Western Australia has been undertaking the world’s largest bullet lead study as part of her PhD. “Around 20% of homicides and armed robberies in Australia involve the use of a gun,” she said. “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.”

Bradley’s study builds on research the FBI started when US President John F. Kennedy was assassinated in 1963. The FBI only determined seven elements present in bullet lead, and dropped compositional analysis in 2005 because it lacked robustness.

Bradley has revisited the technique, enlisting two Australian ammunition manufacturers – each with different ways of making bullets – to test her hypothesis. After shooting the heads of slaughtered pigs 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.

Bradley found that the elemental signature of bullets remains unchanged throughout the manufacturing process. By determining up to 19 trace elements found in bullet lead – including arsenic, gold and mercury – Bradley can now trace a bullet back to its batch of origin, no matter where it was manufactured or where the lead was sourced.

The Western Australia Police Service also provided reference ammunition to build up a database of different bullets. “The ammunition I was most excited about analysing was three boxes of unopened military cartridges from 1942, wrapped in twine and date-stamped,” Bradley said. “We found they had many similarities with modern ammunition, not surprisingly, as bullets are often made from recycled lead.”