ASU study reveals information on inconclusive gun evidence

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In court cases, inconclusive decisions on cartridge-case comparisons from guns can be a contributing factor in putting innocent people behind bars, and the lack of rigorous research in the field of forensic ballistics can keep them there. 

A psychology professor at ASU’s new School of Interdisciplinary Forensics, Stephanie Madon, was the principal investigator in this new study about gun forensics.

“Some guns use ammunition in which the bullet is encased within a jacket, and firearm examiners, sometimes they’ll be examining the bullet, other times they’ll focus on that jacket, which is called a cartridge case,” Madon said. “What happens is as the bullet leaves the gun, and the gun will leave tool marks on the bullet; it will also leave tool marks on the cartridge case, which is ultimately ejected from the gun and lands on the ground and can be recovered from the crime scene.”

The study shows 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. This can help provide wrongly convicted people with information in their post-conviction arguments.

“Those tool marks that appear on cartridge cases you can think of like the fingerprint or the calling card of the gun. What firearm examiners do is they will take cartridge cases, let’s say some recovered from a crime scene, some test fired from a suspect’s gun, and they’ll examine them under a microscope to see if the tool marks are lining up, and if they are, they’ll say they match; they came from the same gun,” Madon said.

Madon said when forensic results are deemed inconclusive, it means there weren’t enough tool marks to prove the bullets came from the same gun, and there weren’t enough mismatches to say the bullets were from different guns.

“What our study showed is that inconclusive evidence or inconclusive decisions are pointing towards innocence more than guilt. Inconclusive are a bit of a troubling problem in the field,” Madon said.

Dr. Stephanie Madon, Psychology Professor at ASU School of Interdisciplinary Forensics

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