Study finds less than 1.2 percent error rate in matching bullets fired from Glock semiautomatic pistol barrels to the actual firearm.

The NIJ-funded study described in "Study Identifies Ways to Improve ATF Ballistic Evidence Program" looked at the operation of the National Integrated Ballistic Information Network (NIBIN), not at the underlying science of firearm and tool mark examination. This forensic science — sometimes referred to by laypeople as "ballistics" — is concerned with the validity of matching a fired bullet to a particular firearm.

So what is the current state of the science of firearm and tool mark examinations? Are these examinations accurate, reliable and valid?

First, the basics: Firearms have numerous metal parts. During the manufacture of a firearm, the machining process leaves unique, microscopic markings (called tool marks) on some of these parts. When most firearms are fired, these tool marks are transferred to the discharged ("spent") cartridge casings and bullets. This evidence can be collected from the scene of a crime, such as a homicide or shooting, and firearm and tool mark examiners can compare them with a test-fired firearm that, for example, has been confiscated from a suspect.

Since 2009, NIJ has funded research to determine the accuracy and reliability of firearms examinations — that is, whether a fired bullet (sometimes referred to as a spent projectile) was ejected from a particular firearm or the probability of finding unique patterns on casings that are shared by spent ammunition from the same firearm. NIJ's most recent findings, released in February 2014, established an error rate of less than 1.2 percent in matching bullets fired from Glock semiautomatic pistol barrels to the actual firearm.

The study — a collaboration between a Florida International University statistician and the Miami-Dade Police Department, which has been studying Glock barrels since 1994 — was designed to answer two basic questions:

• Will trained firearm and tool mark examiners looking at bullets fired through consecutively manufactured firearm barrels that contain the same barcode-like pattern be able to correctly identify the firearm that fired the bullet?
• What role does an examiner's level of experience play in accurately identifying the firearm that fired an unknown (or "questioned") bullet?

The experiment looked at bullets fired from 10 consecutively manufactured Glock barrels. Here's the interesting part: During the manufacturing process, specific Glock barrels are imprinted with a barcodelike pattern called the Enhanced Bullet Identification System (EBIS). The idea behind this study was that even though these barrels were consecutively made and cut with the same EBIS pattern, their "signatures" (or tool marks) should still be different. Consecutively manufactured barrels, as the final report states, "represent the best possibility for the production of two firearms that could produce non-distinguishable markings," since the same tools and machining processes were used, back to back, on one barrel after another.

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