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Gunshot residue (GSR) is principally composed of burnt and unburnt particles from the explosive primer, the propellant, as well as components from the bullet, the cartridge case and the firearm used. There are authors who use other definitions, such as cartridge discharge residue (CDR) or firearm discharge residue (FDR). Gunshot Residue (GSR) is the residue that gets deposited on the hands of the shooter after the bullet has been fired. It may or may not have some burnt, unburnt or semi-burnt particles. Gunshot residue can travel out from the gun to distances of 3–5 feet (0.9–1.5 meters) or even farther. At the farthest distance, only a few trace particles may be present. This information can be useful in determining if someone was near a gun while it was discharged. GSR can match the residue left on clothes to match a specific gun.
In 1971 John Boehm presented some micrographs of GSR particles found during the examination of bullet entrance holes using a scanning electron microscope. If the scanning electron microscope is equipped with an energy-dispersive X-ray spectroscopy detector, the chemical elements present in such particles, mainly lead, antimony and barium, can be identified.
In 1979 Wolten et al. proposed a classification of GSR following compositional criteria, morphology and size. Four compositions were considered “characteristic”: lead, antimony and barium; barium, calcium and silicon; antimony and barium. The authors proposed some rules about the chemical elements which could also be present in these particles.
Wallace and McQuillan published a new classification of the GSR particles in 1984. They called “unique” particles the ones containing lead, antimony and barium, or the ones containing antimony and barium. Also for Wallace and McQuillan in these particles only some chemical elements could be present.
In the latest ASTM Standard Guide for GSR analysis by Scanning Electron Microscopy/Energy Dispersive X-ray Spectrometry (SEM-EDX) particles containing lead, antimony and barium, and respecting some rules related to the morphology and to the presence of other elements are considered characteristic of GSR. The most definitive method to determine if a particle is characteristic of or consistent with GSR is by its elemental profile. An approach to the identification of particles characteristic of or consistent with GSR is to compare the elemental profile of the recovered particulate with that collected from case-specific known source items, such as the recovered weapon, cartridge cases or victim-related items whenever necessary. This approach was called ‘‘case by case’’ by Romolo and Margot in an article published in 2001. In 2010 Dalby et al. published the latest review on the subject and concluded that the adoption of a ‘‘case by case’’ approach to GSR analysis must be seen as preferable, in agreement with Romolo and Margot.
In light of similar particles which can be produced from extraneous sources, both Mosher et al. (1998) and Grima et al. (2012) presented evidence of pyrotechnic particles which can be mistakenly identified as GSR. Both publications highlight that certain markers of exclusion and reference to the general population of collected particulate can help the expert in designating GSR-similar particles as firework-sourced.
Particle analysis by scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy detector can be the most powerful tool for forensic scientists to determine the proximity to a discharging firearm and/or the contact with a surface exposed to GSR (firearm, spent cartridge case, target hole), if proper attention is paid to avoid secondary gunshot residue transfer from officers onto subjects or items to be tested for GSR, and to avoid contamination in the laboratories.
The two main groups of specialists currently active on gunshot residue analysis are the Scientific Working Group for Gunshot Residue (SWGGSR) based in USA and the ENFSI EWG Firearms/GSR Working Group based in Europe.
A positive result for GSR from SEM-EDX analysis can mean many things. Mainly it indicates that the person sampled was either in the vicinity of a gun when it was fired, handled a gun after it was fired, or touched something that was around the gun when it was fired. (For example: When a person goes to the aid of a victim of a gunshot wound, some GSR particles can be transferred from the victim.)
A negative result can mean that the person was nowhere near the gun when it was fired, that they were near it but not close enough for GSR to land on them or it can mean that the GSR deposited on them has worn off. GSR is the consistency of flour and typically only stays on hands of a living person for a maximum of 4-6 hours. Wiping the hands on anything, even putting them in and out of pockets can transfer GSR off the hands. Victims don't always get GSR on them, even suicide victims can test negative for GSR.
Unless the ammunition used was specifically tagged in some way, it is not possible to identify a weapon used in a crime only by the presence of GSR on the weapon. Bullets can be matched back to a gun using ballistics, but matching the gun shot residue on a suspect or victim to a specific gun is not possible at this time.