2022
Veronica Franklin, M.S.
Atlanta, GA
Learning objectives: Attendees will learn how subclass characteristics persists after a breech face is finished with tumbling or glass bead blasting. They will also learn of a graphical user interface we intend to use for firearm examiners to mark-up images of cartridge cases and breech faces.
Proposition: Subclass characteristics produced on breech faces when manufactured by a broach, lathe, and endmill, may persist after being finished with tumbling but unlikely to presist after being finished with glass bead blasting. Introduce the graphical user interface that we are developing for the examiner portion of this study.
Abstract:
Subclass characteristics are caused by damage, defects, and/or wear on a cutting tool that transfers to a small group of consecutively manufactured surfaces, but not all surfaces made by that tool. They are more distinctive than class characteristics, but unlike individual characteristics they cannot be used to identify a cartridge case as having been fired by a particular firearm. If firearm examiners mistake these characteristics as being individual when they are subclass and if such marks are used for comparison, a false identification may result. This project is a continuation of a preliminary study regarding the formation of subclass characteristics during the manufacturing process of breech face inserts. The objective of this part of the study is to assess the persistence of subclass characteristics after the breech face inserts have been finished by tumbling with ceramic media or glass bead blasting. Another objective is to observe the impression of subclass characteristics to cartridge cases during firing. We propose subclass characteristics can be produced when breech faces are manufactured using a broach, lathe, or end mill and that such subclass characteristics will transfer to the fired cartridge cases as part of the breech face impression prior to the breech face being finished but not necessarily after
such a finishing process is complete.
Thirty breech faces, ten of each method mentioned earlier were produced consecutively for a Thompson Center (T/C®) Contender® G2 pistol. After manufacturing, each breech face was scanned using a Sensofar® S neox confocal microscope (10x objective, Confocal Fusion). Ten test fires were collected for each breech face and scans of all test fires were collected to observe the transfer of subclass in the breech face impression. Once the 3D scans were collected, NIST’s congruent matching cells (CMC) algorithm was used to compare all test fires. This comparison was used to assess the effect of subclass on CMC comparison performance. Five breech face inserts were randomly selected from each manufacturing method and were tumbled in abrasive tumbling media for ten hours. The other five breech face inserts were finished by glass bead blasting. The topographic scans of the finished breech face inserts were collected. Subsequently, ten test fires were collected and scanned for CMC comparison. These 1 This project is funded by CSAFE under CSAFE F&T II. Subclass Characterization and Analysis of Firearms.
Test fires were compared to the unfinished breech face test fires. These comparisons were used to assess if the subclass characteristics persisted after the selected finishing method. Results of this study have shown that subclass characteristics can be produced using these methods. Although the methods of production are very similar to those used by a firearms manufacturer, they are not perfectly identical. They do 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. A further objective of this presentation is to introduce a graphical user interface (GUI), which is in the process of development, that will allow examiners to mark-up subclass characteristics – and other characteristics. This GUI will allow firearm examiners to assist this study by reviewing images of the manufactured breech face inserts and test fires. These data will be used to develop an algorithm that will attempt to identify potential subclass characteristics on breech face impressions and of a questioned cartridge case.