Evaluation of gunshot residue (GSR) evidence: Surveys of prevalence of GSR on clothing and frequency of residue types

The evaluative approach is a logical approach to interpreting scientific findings in criminal cases, applying knowledge regarding the transfer, persistence and recovery of particulate material. The application of this approach to interpreting the finding of gunshot residue on the clothing of a suspect requires knowledge of background levels of GSR on clothing and on the frequency of different residue types in a particular environment. The cuffs of 100 upper outer garments submitted to a forensic laboratory in connection with non-firearms offences were sampled for gunshot residue. No 3-component lead/antimony/barium particles were found on 98 of them. Two 3-component particles were found on one of them and one 3-component particle was found on another. The frequency of occurrence of various particle types regarded as consistent with GSR was also explored. The findings show that, while 3-component particles were somewhat more likely to be encountered by chance on clothing than on hands, they are still relatively uncommon events. To investigate the frequency of occurrence of particular residue types, 100 discharged rounds of ammunition recovered at crime scenes were sampled and the types of residue present were determined. The results show that some residue types are significantly more common than others. Both sets of data will be of value in evaluating the significance of finding GSR on clothing of suspects in criminal cases.