Estimation of changes in breech face and firing pin marks over consecutive discharges and its impact on an IBIS ® Heritage™ System.

When a firearm is discharged, the individual marks of the breech face and firing pin are imprinted onto the primer of the cartridge case. These individual marks are reproducible between shots; however, over a large number of consecutive shots, it has been observed that minute changes in these individual marks may occur. Changes in individual marks may affect an examiner in their ability to identify or eliminate and may change the magnitude of a likelihood ratio, depending on the system used by the laboratory, but the effect that these changes have on the Integrated Ballistic Identification System (IBIS® ) is largely unknown. If such changes negatively affect the performance of the IBIS® then consequences may result with respect to the correct matching candidate not be returned in the top results further compared by an examiner. Two hundred consecutive test fires performed in a clean environment (indoor shooting range) were collected in sequence from 24 new Ruger® SR9 9mm pistols and entered into an IBIS® Heritage™ System. The full known match data were extracted for each firearm, and δ sets were created that had a specific number of cartridge cases between the two cartridge cases being compared. Receiver operating characteristic (ROC) area under the curve (AUC) values for these δ sets were compared to the full known match sets to determine if any significant changes in performance resulted. Although there were instances of significant differences, these only occurred in less than 25% of comparisons, and overall no decreasing trends in performance were observed.