Forensic Engineering Analysis of Ejected Cartridge Case Patterns for the Reconstruction of Firearms-Related Incidents

Firearms-related incidents present an important area of study for forensic engineers. Analysis of bullet trajectories, examination of firearms failures, and the study of wound dynamics and projectile injuries are some of the topics to which the forensic engineer can offer valuable knowledge and insight for the purpose of reconstructing a shooting incident in a forensic context. One specific topic of study that forensic engineers engaging in the reconstruction of firearms-related incidents should clearly understand is the nature and distribution of ejected cartridge case patterns left at a crime scene after the discharge of semi-automatic firearms. While studies have been conducted into the effects that firearm design, ammunition, grip, stance, and movement have on the distribution of ejected cartridge cases, few, if any, of these studies have involved any type of engineering analysis. This paper will address the nature of engineering analysis applicable to the distribution of ejected cartridge cases at a crime scene by specifically addressing mechanical issues related to the design and maintenance of the firearm itself, the flight characteristics of ejected cartridge cases, and the restitution of cartridge cases that collide with intermediary objects. Experimental results will be presented, and testing methodologies will be discussed. Methods for simulating cartridge case ejection patterns based on testing data using Monte Carlo methods will be covered. Application of testing data and simulation to actual casework will be demonstrated.