Pressure factors in cartridge case comparisons

As submitted by the proposer:

Previous work has indicated that the pairwise comparison of cartridge cases fired through the same firearm and compared using the Integrated Ballistics Identification System (IBIS) results in a wide-range of breech face and firing pin scores. Score variability is a result of the variability in the markings on cartridge cases fired by the same firearm. A clear understanding of the factors which influence the quality of breech face and firing pin impressions on a cartridge case during a particular discharge of a firearm will provide a fundamental understanding of variability in test cartridge cases and how such cartridge cases are compared with those recovered from a crime scene. Defining and assessing the variables which affect the formation of these impressions is necessary in modern day forensic science.

In this project, a simulation study will be undertaken to assess the effects of these variances on chamber pressures. The effects of the physical dimensions of the bullets and cartridge cases on the manufacturing process will be assessed by reloading cartridges with various bullet weights, propellants, and primers. A firearm which can be fitted with a variety of caliber barrels will be used to cover a broad chamber pressure range. All of the ammunition thus fired will have the same breech face and firing pin impressions irrespective of the caliber of the ammunition.

All cartridge cases will then be entered into the IBIS under the same caliber designation and all cartridge cases will be correlated against each other. The generated breech face and firing pin scores, together with the calculated and measured muzzle velocities and chamber pressures, will be used to assess the performance of the system.

Classical statistical analyses will be used to assess the effects of significantly different chamber pressures in compared pairs of cartridge cases. The result of this research will allow firearms examiners, from a fundamental perspective, to understand the effects of variability in markings produced by the discharge of a single firearm; assess the influence of differences in chamber pressure between crime scene cartridge cases and test fired cartridge cases; provide guidance on the variability of performance of manufactured ammunition as it pertains to comparisons; and assess the variation between primers from different manufacturers.

Note: This project contains a research and/or development component, as defined in applicable law, and complies with Part 200 Uniform Requirements - 2 CFR 200.210(a)(14).

ca/ncf