DNA transfer to firearms in alternative realistic handling scenarios

Firearms are the most relevant items of evidence in gun-related crimes, likely bearing various traces facilitating an objective reconstruction of the crime. Trace DNA recovered from firearm surfaces might help to identify individual(s) having handled the firearm and thereby possibly to link the firearm and the corresponding shooter, however, the interpretation of DNA traces on handled items can be challenging and requires a detailed understanding of various factors impacting DNA prevalence, transfer, persistence and recovery. Herein, we aimed at improving our understanding of factors affecting the variability of trace DNA characteristics recovered from firearms handled in gun-related crimes: Skin contact traces were recovered from various outer surfaces of two types of firearms handled in four realistic, casework-relevant handling scenarios and the corresponding trace characteristics (DNA yield, number of contributors, relative profile contribution for known and unknown contributors, LRs) were compared. Trace DNA characteristics differed distinctly between handling conditions, firearm and surface types as well as handling individuals and intraindividual deposits emphasizing the variability and complexity of trace DNA profile composition expected to be recovered from firearms after realistic handling scenarios. The obtained results can provide useful insights for forensic experts evaluating alternative activity level propositions in gun-related crimes.