FIRED CARTRIDGE CASE EJECTION PATTERNS FROM SEMI-AUTOMATIC FIREARMS

Abstract

During testimony, “experts” often cite that spent cartridge case ejection locations from a semi-automatic firearm indicate the location of the shooter based on the assumption that most spent cartridge cases land to the right and rear of the shooter. The authors of this study investigated whether spent cartridge case ejection locations are an accurate indicator of a shooter’s location. Eight different semi-automatic weapons most frequently used by police officers were used to collect data from eleven different shooting positions. The results highlighted the significant inconsistency of the spent

cartridge case ejection locations that occurred across test positions even when several factors including firearm type, firearm position, and ammunition were accounted for. Of 7,670 bullets fired, over 25 percent of the spent cartridge casings landed somewhere other than to the right and rear of the shooter where it is commonly accepted they should land. That pattern inconsistency is significant and demonstrates that determining shooter

location from the spent cartridge case alone leads to only a tentative estimate of the shooter’s location.