ForensicsPutting statistics into forensic firearms identification

Published 9 February 2018

When a gun is fired, and the bullet blasts down the barrel, it encounters ridges and grooves that cause it to spin, increasing the accuracy of the shot. Those ridges dig into the soft metal of the bullet, leaving striations. At the same time that the bullet explodes forward, the cartridge case explodes backward with equal force against the mechanism that absorbs the recoil, called the breech face. This stamps an impression of the breech face into the soft metal at the base of the cartridge case, which is then ejected from the gun. Researchers have developed a statistical approach for ballistic comparisons that may enable numerical testimony – similar to a DNA expert expressing the strength of the evidence numerically when testifying about genetic evidence.

On 14 February 1929, gunmen working for Al Capone disguised themselves as police officers, entered the warehouse of a competing gang, and shot seven of their rivals dead. The St. Valentine’s Day Massacre is famous not only in the annals of gangland history, but also the history of forensic science. Capone denied involvement, but an early forensic scientist named Calvin Goddard linked bullets from the crime scene to Tommy guns found at the home of one of Capone’s men. Although the case never made it to trial—and Capone’s involvement was never proved in a court of law—media coverage introduced millions of readers to Goddard and his strange-looking microscope.

That microscope had a split screen that allowed Goddard to compare bullets or cartridge cases, the metal cases a gun ejects after firing a bullet, side by side. If markings on the bullets or cases matched, that indicated that they were fired from the same gun. Firearms examiners still use that same method today, but it has an important limitation: After visually comparing two bullets or cartridge cases, the examiner can offer an expert opinion as to whether they match. But they cannot express the strength of the evidence numerically, the way a DNA expert can when testifying about genetic evidence.

NIST says that now, a team of researchers at the National Institute of Standards and Technology (NIST) has developed a statistical approach for ballistic comparisons that may enable numerical testimony. While other research groups are also working on this problem, the advantages of the NIST approach include a low error rate in initial tests and that it is relatively easy to explain to a jury. The researchers described their approach in Forensic Science Internationa.

When comparing two cartridge cases, the NIST method produces a numerical score that describes how similar they are. It also estimates the probability that random effects might cause a false positive match—a concept similar to match probabilities for DNA evidence.

“No scientific method has a zero error rate,” said John Song, a NIST mechanical engineer and the lead author of the study. “Our goal is to give the examiner a way to estimate the probability of this type of error so the jury can take that into account when deciding guilt or innocence.”