The brand of bullets used in a crime could be identified even when no bullets are found at the scene, by analysing tiny metal scraps they left behind.
Forensics experts may attempt to link a suspect to a crime by analysing bullets or spent bullet casings found at the crime scene and demonstrating they were fired by the suspect’s gun. But doing so when the bullets are not present at the scene – for instance, when they have been removed by the perpetrator – is far more challenging.  at Texas Tech University and his colleagues say a tried-and-true method from analytical chemistry can help.
Their idea was to focus on the residue left by the bullet – for instance, tiny metal shards deposited on furniture or a wall as the bullet ricochets off surfaces. The relative concentration of lead and antimony in the residue could then be matched to a bullet manufacturer, based on the assumption that each manufacturer produces bullets with a subtly unique chemical “fingerprint”.
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To test their idea, the researchers collected 250 bullets from 10 different manufacturers, removed each bullet from its casing and washed it with water and alcohol to remove any dirt. Then they scraped a 1-milligram sample from each bullet and used a method called atomic absorption spectroscopy (AAS) to determine how much of the two metals was in each sample.
This method uses the fact that different elements absorb light at different frequencies when heated, and it can detect an element even if it is present as just one part per billion in a sample. “This is a very established method, my students learn it in a few hours,” says Halámek.
The analysis confirmed that it was possible to identify a bullet’s manufacturer from a tiny sample – similar to what could be recovered from a surface at a crime scene that a bullet had ricocheted off. But Halámek says the method would be more useful if bullet manufacturers started recording the chemical composition of their bullets in a publicly available database.
However, other researchers question how useful the approach would be in the real world. at Iowa State University says there can be coincidental matches if bullets from different manufacturers have similar compositions, as well as unexpected variation among bullets from the same manufacturer, which can diminish the reliability of this approach.
Halámek says these issues, which , could be addressed by having bullet manufacturers analyse and share data for each batch of bullets, and implement better quality control.
But at the University of Dundee, UK, says this could be very challenging in practice.
ACS Omega
