The 2 stages of cartridge primer toolmark production and the implied impact of cartridge manufacturing tolerances

Abstract

Many methods of ballistic toolmark comparison rely upon comparison using 2D greyscale imaging. However, newly emerging analysis methods such as areal surface analysis now utilise an extra dimension of measurement allowing the surface heights/depths of unique toolmark features to be recorded in a densely populated (x,y,z) array for a 3D/areal quantitative comparative analysis. Due to this step change, the colloquialism in referring to the crater produced at the centre of the primer during firing as a “firing pin impression” has become a misnomer, leading some to believe that this toolmark is produced via a single process, where the critical variable is the condition of the firing pin. Furthermore, current forensic ballistic methodology relies on the microscopic differences between individual fired bullets and cartridge cases produced as a result of the manufacturing process of a particular firearm, in this case “matched toolmarks” confirm a ballistic match to a specific firearm. However, very rarely is it considered that the ammunition itself possesses minute differences produced during manufacture that could affect the ballistic match efficacy. This study examines the discharge process of conventional centrefire ammunition and concludes that the unique toolmarks upon the cartridge primer are definitively produced in two defined stages. This conclusion suggests that the factory loading and quality control tolerances of the cartridge itself should now be considered to be a more significant contributing factor to the production of cartridge primer toolmarks than has previously been accepted.