An Investigation of DNA Recovery from Firearms and Cartridge Cases

ABSTRACT This study examines the relative success rates of DNA recovery from firearm locations normally handled during the process of loading and discharging the firearm, ejected cartridge cases, and unspent cartridges to aid in the association of the firearm, ammunition, or cartridge case to a potential perpetrator by means of DNA evidence. Four firearms were selected for study: a revolver, a pistol, a break-action shotgun and a pump-action shotgun. Thirty-seven percent (37%, 47/126) of the experimental samples from the exterior locations of the four firearms tested yielded DNA quantities suitable for further processing (≥ 240 pg), with an average yield of 1.1 ng of DNA per sample. DNA recovery was most successful from the grip of all four firearms (80% of trials) and the slide serrations of the pistol (87.5% of trials). An association between the shooter and the DNA profile obtained was established in 30% of the samples. The DNA profiles obtained were either of single source or mixtures from which the shooter could not be excluded as a contributor. In most cases, the shooter could not be excluded as the major contributor to the mixture; however, there were instances where the shooter could only be attributed to the mixture as a minor contributor of DNA. The remaining 70% of samples did not yield sufficient DNA or the profiles obtained were partial or complex mixtures. DNA recovery from samples taken from fired cartridge cases, unfired cartridges, and the top and sides of the magazine of a pistol were evaluated in a separate study. Only two of the 72 experimental samples, a sample from an ejected cartridge case and a sample from the magazine, yielded DNA quantities above the 240 pg amplification threshold. Only the magazine sample yielded a full 9 STR locus DNA profile that was attributable to the shooter. An additional experiment was conducted to examine the effects of heat, gases, and mechanical action on DNA recovery. Cellular material was deposited (via saliva) on the exterior of the ammunition loaded into the firearm. Samples from the cartridges cases discharged from the firearm and loaded unfired cartridges both demonstrated loss and degradation of DNA. DNA recovery from cartridges and ejected cartridge cases has extremely limited success, compounded by the act of discharging the firearm that compromises the quality of the DNA as a template for amplification.