Effect of Subclass Characteristics on Congruent Matching Cells (CMC) Algorithm

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Franklin, Veronica
Morris, Keith
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Center for Statistics and Applications in Forensic Evidence
This presentation is a continuation of subclass characteristics present on consecutively manufactured breech face inserts. The objective is to assess manufacturing methods similar to those used by firearm manufacturers. It has been shown that subclass characteristics will be present on breech faces initially manufactured by broaching and plunge milling. The face milling procedure of breech faces is proposed to also produce subclass characteristics. Turned breech faces have not shown subclass characteristics in the preliminary study.
Twenty breech faces of each of the methods mentioned were consecutively produced for a Thompson Center (T/C®) Contender® G2 pistol. After manufacture, the 3D topography of each breech face was collected with a Sensofar® S neox confocal microscope (10x objective, Confocal Fusion). The CMC algorithm was used to compare all the breech face inserts in a pairwise manner. The results were used to assess the presence of subclass characteristics and the effect they may have on the performance of the algorithm. Results have shown that subclass characteristics have been produced using the chosen manufacturing methods. It should be noted that the manufacturing methods are very similar to those used by a firearms manufacturer but are obviously not identical.
Another objective of this project is to have firearms examiners assess the various manufacturing techniques by microscopical examination of our samples. Double cast reproductions of each of the manufactured breech face inserts have been produced. These reproductions were compared by microscopic examination and the CMC algorithm.
The purpose of double cast reproductions is to provide identical samples for comparison by multiple examiners. This technique has been used produce almost identical samples for examination by members of the subclass characteristics advisory group of this study. They will provide input on the manufacturing techniques and determine if and where subclass characteristics are present on the breech faces. Results from the comparison of the double cast reproductions and their master breech faces via CMC will be discussed.
The following was presented at the Association of Firearm and Tool Mark Examiners (AFTE) 2023, Austin, Texas, May 21-26, 2023. Posted with permission of CSAFE.

Copyright 2023, The Authors.