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Study: ‘Inconclusive Finding’ by Examiner of Cartridge Casing Should Be Finding of ‘Excluded’ 85% of the Time

by Douglas Ankney

A recent study by researchers from Arizona State University (“ASU”) “found that 85% of cartridge cases that were judged inconclusive by forensic firearm examiners were actually fired by two different guns. In an actual crime scene investigation, that would mean that the cartridge cases did not match the gun in question.”

The study, published in the Proceedings of the National Academy of Sciences, “was the first to closely examine the usefulness of inconclusive decisions in helping to clear the innocent of wrongdoing,” said Max Guyll, an associate professor of psychology at ASU’s new School of Interdisciplinary Forensics and one of the researchers behind the study. “What we found was that inconclusive decisions were predictive of, or related to, cartridges that are fired from different guns.”

When a bullet is fired from a gun, firearm examiners claim that distinctive markings are stamped onto the cartridge casing from the gun’s firing pin, breach face, extractor, slide mechanism, and wear and tear on these surfaces via ordinary use. The examiners claim that these distinctive markings can be used to determine if a cartridge casing recovered from a crime scene was ejected from a particular firearm attributed to the defendant; that is, either the cartridge’s markings “match” the markings unique to the firearm in question or they do not match. But in some cases, the markings are such that the examiner cannot make a determination one way or the other. These findings are reported as “inconclusive.”

The study was based on 1,811 findings made by 228 trained firearms examiners who had examined cartridge cases left by 7,244 bullets fired from 28 guns. Each examiner was provided with eight sets of four cartridge cases, with three cases in each set coming from the same gun. The fourth case may or may not have come from the same gun.

The study also revealed that examiners rendered an “inconclusive” finding 20% of the time. And 35% of cartridge casings fired from different guns were judged inconclusive compared with only 6.5% of casings fired from the same gun were judged inconclusive. Importantly, at trial, testimony that the examiner’s results were “inconclusive” fails to communicate to jurors that the finding was more likely exculpatory than inculpatory. “You don’t want to be an innocent guy sitting in a courtroom when results come back inconclusive,” Guyll said.  


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