Experimental evaluation of the impact of seating depth variations on observed marks on primers

Although the examination of bullet and primer striae patterns is still a very important task in forensic ballistics, a fundamental explanation or theoretical model for reasoning about the (ir)reproducibility of the observed marks is still non-existing. More specifically, it is still unclear which ammunition production or firing condition parameters may play an important role in the striae and marks formation process. From a practical point of view this situation is unfortunate because a proper knowledge of these parameters and conditions could help underpin and explain certain differences found in actual casework investigations. Additionally, a proper understanding of these issues could allow examiners to select or develop a near-optimal test firing procedure by using more detailed and controlled variations of certain intra- and/or inter-ammunition parameters than the ones that are in use today. In this paper, we discuss the results of an experiment aimed at evaluating the impact of primer seating depth variations on observed primer striae and other marks. First, three sets of cartridge cases with different seating depth intervals were selected. Second, each of these sets were fired consecutively using a selected pistol. Finally, the cartridge cases were examined using both traditional optical microscopes and three-dimensional (3D) profilometer techniques. The results reported in this paper show that in our experiments no important impact of the initial primer seating depth on the observed primer striae patterns could be found, but that the firing pin impression does seem to change if the initial seating depth is larger than a certain threshold.