Firearms, particularly pistols, are commonly used in violent crimes, though the actual weapon used is rarely recovered. Nevertheless, spent cartridge casings ejected during shooting are often left at the scene and recovered by law enforcement. These casings may contain DNA deposited by the loader of the firearm, who could potentially be identified using short tandem repeat (STR) analysis. However, DNA recovered from spent casings is often degraded and present in low copy numbers. Owing to this, crime laboratories have had limited STR typing success from casings, thus, it is essential that methods for DNA recovery and analysis be optimized. Multiple variables, such as swabbing or soaking casings, pre-treatment of soaking vessels, shaking casings during soaking, pre-digestion incubation of soaked samples, and the duration of digestion with concurrent shaking were examined, with the goal of optimizing methods to improve DNA yields. Volunteers loaded cartridges into the magazine of a pistol, cartridges were fired, casings were collected, DNAs were recovered and extracted with one of five optimized methods (double swab or soak with an organic extraction, double swab or soak with a silica-based extraction, or single swab with a non-binding DNA extraction), DNAs were quantified, and amplified with AmpFℓSTR® MiniFilerTM and/or PowerPlex® Fusion. Comparisons of DNA yields and STR profiles demonstrated double swabbing with organic extraction and amplification with Fusion generated significantly more DNA and alleles consistent with the loader. Ultimately, optimization of protocols for DNA recovery and analysis from spent cartridge casings generated a significant increase in loader STR data.
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