Semester

Summer

Date of Graduation

2025

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Forensic and Investigative Science

Committee Chair

Keith Morris

Committee Member

Tatiana Trejos

Committee Member

Eric Law

Abstract

Forensic firearm analysis involves the comparison of test fires from a suspect firearm to an unknown sample, typically collected from a crime scene. Test fires and the unknown sample are examined for agreement in class characteristics and sufficient agreement in individual characteristics using a comparison microscope. The forensic analysis of firearms has been challenged both in court and by governmental organizations for its subjective nature and the inability to accurately report error rates.

To combat these challenges, the National Institute of Standards and Technology (NIST) has proposed using computer-based algorithms, such as Congruent Matching Cells (CMC). These CMC algorithm calculates the degree of similarity between samples using topography measurements. Instead of using the Association of Firearm and Tool Mark (AFTE) Theory of Identification, the number of congruent matching cells determines if the unknown sample and test fires were discharged by the same firearm.

Two shotguns, a Mossberg® 500® and a Remington® 870TM each discharged 405 shotshells with varying pellet weight, pellet size, and charge weight. Shotshells were then scanned with a confocal microscope prior to being imported into the CMC algorithm for cropping and comparisons. Comparisons were only performed within the same firearm and were analyzed to determine applicability of the CMC algorithm in regard to shotshell comparisons as well as to determine if there was any influence by pellet weight, pellet size, and charge weight on the breech face impressions.

The results of this study demonstrated that the CMC algorithm can be applied to shotshell comparisons. In order to improve the utility of the algorithm the congruency thresholds need to be optimized for such firearms. The area of the firing pin impressions on shotshells are very large in comparison to area of the primer surface. This resulted in high false negative rates for both firearms (FNR Mossberg®500® = 48.4% and FNR Remington®870TM = 38.9%) when using the CMC threshold of six (the threshold determined for pistols). The results show that charge weight, pellet size, and pellet weight all affect the CMC count and CMCpercentages due to the variation in breech face impressions as a result of their role in the cycle of fire.

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