By Brian Carnell

Monday, April 22, 2002

New Scientist reported this month on an investigation into a common technique used to match bullets found at crime scenes with bullets owned by suspects. Testimony using this technique has been used at hundreds of trials in the United States and United Kingdom, but it turns out there is no scientific basis for the bullet matching technique.

The most reliable bullet test compares the rifling on a bullet found at a crime scene with the rifling from a bullet fired from a known gun. Sometimes, however, rifling tests are not possible or inconclusive. So forensic scientists often conduct a chemical matching of bullets.

The idea behind this is that the amount of trace elements in the lead used to pour the bullet are going to vary from batch to batch. So forensic experts take a bullet found at the scene of a crime and test if for trace amounts of antimony, tin, arsenic, copper, bismuth, silver and cadmium. Then they test bullets taken from a suspect for the exact same trace elements.

If the levels of trace elements match, then the conclusion is that these two bullets came from the same batch and the forensic expert will testify as such. This does not prove that the suspect fired the bullet, but does provide strong circumstantial evidence that ties him or her to the bullet.

The problem, it turns out, is that forensic scientists never bothered to corroborate the accuracy of this test, and it turns out to be wildly inaccurate. Erik Randich, a forensics consultant and metallurgist at Lawrence Livermore National Laboratory, and Bill Tobin, a former FBI chief metallurgist, examined records over a period of two years at two companies that produce lead. Because there are strict requirements for allowable trace elements in lead due to its common use in automobile batteries, lead companies routinely test samples from batches for quality control purposes.

Randich and Tobin found that it was impossible to tell whether or not two bullets had been produced from the same batch of lead. On the one hand, the composition of trace elements differed in the same batch at the beginning of the day as compared to at the end of the day, probably due to the oxidation of the trace elements. Moreover, they found several instances where batches of lead produced months apart contained identical levels of trace elements.

"You'll find bullets that are indistinguishable that were made months apart," Randich told New Scientist.

This finding agrees with a May 2000 report on the technique that the FBI commissioned which found that it was theoretically possible to match bullets from the same batch, but that the FBI lacked the data to determine how likely it was that bullets with similar chemical compositions came from the same batch of lead.

Sources:

Chemical matching of bullets fatally flawed. New Scientists, April 17, 2002.

Bullet fingerprinting flawed, says scientist. The BBC, April 17, 2002.

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