The memory effect of gunshot residues on discharged semi-automatic pistols

• Main Authors: Tsan-Peng Mao, 茆璨鵬
• Other Authors: Hsien-Hui Meng
• Language: zh-TW
• Published: 1030
• Online Access: http://ndltd.ncl.edu.tw/handle/u4f657

碩士 === 中央警察大學 === 鑑識科學研究所 === 102 === Abstract Gunshot residues (GSR) are important evidence for the investigation and reconstruction of shooting incidents. They can not only be used to confirm or rebut the confessions of suspects but also to determine whether suspected individuals or objects were present at crime scenes or not. The GSR deposited on targets can be employed to confirm suspected bullet holes and to evaluate the distances between targets and the muzzles of fired guns. Scanning electron microscopy combined with energy dispersive X-ray spectrometry (SEM/EDS) is presently the standard approach for forensic analysis of inorganic GSR. Spheroidal micro-particles containing characteristic elements of antimony, barium, and lead are employed to conclude the presence of GSR. However, non-toxic primers have been gradually utilized to substitute heavy-metal-contained non-corrosive primers to prevent environmental pollution. This leads to the difficulty of GSR determination. And a remedy called “case by case” procedure is employed to analyze GSR that requiring further analysis of standards collected from spent cartridge cases or/and fired barrels. A number of works revealed that GSR from previous shootings can be firmly retained on the inner surfaces of chambers and barrels. The retained GSR would affect the elemental composition of GSR generated during the following shootings. The phenomenon is called as “the memory effect.” The memory effect could result in the inconsistency of elemental composition of GSR collected from targets, shooters, and spent cartridge cases. This will further impede the reconstruction of the linkages between evidence, suspects, victims, and crime scenes. In order to realize if GSR memory effects exist in the semi-automatic pistols commonly used in Taiwan, we used 24 pieces of confiscated illegal pistols and shooting training pistols belong to the Central Police University to conduct this study. Five brands and models of illegal pistols including Beretta 92FS, CZ 75B, Glock 19, Vektor CP1, and HK USP Compact were studied in this work, where the first two kinds of pistols have traditional rifling and the rest kinds of pistols have polygonal rifling. Three brands and models of training pistols, that are Smith & Wesson 5904、Walther P99、Smith & Wesson M&P 9C, were also studied. GSR memory samples were collected using tungsten pens to scratch the inner surfaces of chamber, breech and muzzle ends of rifled barrel of each pistol. The samples were carbon-coated and then subjected to SEM/EDS analysis. The analysis results indicated that antimony, barium, and lead, that are characteristic elements of GSR originated from non-corrosive primers, were detected in all samples collected from illegal pistols. Elements of potassium, calcium, silicon, and sulfur that are in consistence with elemental compositions of non-toxic primers were detected in all samples collected from training pistols. The results proved that GSR memories do exist in pistols frequently encountered in Taiwan. The carried-out mechanisms of GSR memories were studied via two procedures. Firstly, unfired bullets were forcedly pushed from the breech ends into the rifled barrels that proved to have antimony, barium, and lead memories for a certain distance and then pushed out from the same ends. Antimony, barium, and lead were detected in samples recovered from the rifling marks of the pushed-into bullets. This proved that the friction between passing bullets and the barrel surface played an important role in carrying out GSR memories retained in the barrel. Test firings discharging cartridges with non-toxic primers were then conducted. Antimony, barium, and lead were detected on samples collected from fired bullets, targets, firers’ webs of shooting hands, and the outer surfaces of spent cartridge cases. This proved that GSR memories can be carried out during the discharge of a pistol, and the carried-out GSR memories could interfere with the interpretation of GSR analysis. Training pistols without antimony, barium, or lead memories were used to fire 20 rounds of cartridges with non-corrosive primers. GSR memory samples were collected from the inner surfaces of chamber, breech and muzzle ends of the barrels after cleaning the fired pistols. Antimony, barium, and lead were detected in all of these samples. This proved that characteristic GSR memories can be formed inside of the pistol barrels after firing a number of cartridges with non-corrosive primers. The results of this study showed that memory effects do exist in the barrels of semi-automatic pistols commonly used in Taiwan. And the memory effects could affect the results of GSR analyses while investigating shooting incidents. Hopefully, the results of this study would be helpful for forensic scientists to conduct GSR analysis and to interpret the analysis results more correctly and precisely. Keywords : firearms examination, gunshot residues, memory effect, semi-automatic pistols, scanning electron microscopy/energy dispersive X-ray spectrometry（SEM/EDS）