AFTE Store - Subclass Characteristics: From Origin to Evaluation

Subclass characteristics, which were first brought to the attention of the firearm/toolmark examiner community-at-large in 1992, have long been recognized (even prior to 1992) as a potential obstacle in making reliable common source determinations. The reason is because subclass characteristics have an appearance similar to individual characteristics but, unlike individual characteristics, can be present on a number of tools created in close proximity with one another. Therefore, if not properly assessed, a tool mark on a questioned piece of evidence could be mistakenly linked back to a tool other than the one that made it based on the correspondence of subclass characteristics. Notably, within the last 25 years there have been some excellent research studies in this area that have helped recognize specific machining methods that have a propensity to leave subclass characteristics as well as providing guidance on how to assess tools and tool marks for subclass influences. This article looks to bring together the early historical context with a basic understanding of manufacturing processes and the various studies that have been performed for the purpose of serving as a single, well referenced resource for the issue of subclass characteristics.