2021
Veronica Franklin
Miami, FL
“Subclass characterics are caused by damage, defects, and/or wear on a cutting tool that transfers to a small group of consectuively manufactured surfaces, but not all surfaced made by that tool. They are more distinctive that class characterisics, but unlike individual characteristics, the cannot be used to identify a cartridge case as having been fired by a particular firearm. Firearm examiners can mistake these characteristics as being individual when they are subclass and if used for comparison can lead to a false identification.
The first objective of this study is to assess the production of subclass characteristics during the manufacturing of breech faces with three different methods. The other objected is to observe how well subclass persists on spent cartridge cases without any finishing methods used on the breech faces and when finishing methods are used. We propose subclass characteristics will be produced when breech faces are manufactured with a broach, lathe, and end mill and the subclass will transfer to the spent cartridge cases in the breech face impression.
Thirty breech faces, ten of each method mentioned earlier will be consecutively produced for a Thompson Center (T/C) Contender G2 pistol. Each breech face will be scanned using a Sensofar S neox confocal microscope after manufacturing. Ten test fires will be collected for each breech face and all test fires will also have 3D scans collected to observe the transfer of subclass in the breech face impression. Once the 3D scancs are collected, NIST’s consecutively matching cells (CMC) algorithm will be used to compare all test fires. This comparison will be used to assess the effect ob subclass on CMC comparison performance. Five breech faces will be selected from each manufacturing method and placed in tumble media as a finishing method. The breech faces will have their topography scans collected and ten more test fires will be collected and scanned for CMC comparison. These test fires will also be compared to the unfinished breech face test fires. This comparison will assess if the subclass persists after the selected finishing method.
Results of another study being conducted by our lab has shown we can preoduce subclass characteristics by using these methods. ALthough the methods of production are very similar to those used in a firearm manfucaturing factory, they are not perfectly identical. It does however allow for the use of machining techniques not typically used during the production of actual items. These breech faces will provide insight to firearm examiners regarding the identification of subclass characteristics and the features to consider when completing comparisons. *This project is funded by CSAFE under CSAFE F&T II. Subclass Characterization and Analysis of Fireams. “