Gunshot residue as evidence
Gunshot residue (GSR) is the residue that is expelled when a firearm is discharged, consisting of burned, partially burned, and unburned particles from the primer and gunpowder. The presence of GSR on a person or object provides strong evidence that a firearm has been used, making it a valuable tool in forensic investigations. GSR can also help forensic scientists estimate the distance from which a shot was fired, as the concentration of residue diminishes with distance from the shooter.
The composition of GSR typically includes key elements such as lead, barium, and antimony, which are found in the primer of ammunition. Forensic analysis often involves microscopic examination and various chemical tests to confirm the presence of these elements. Techniques like the modified Griess test and sodium rhodizonate test are used to identify nitrites and lead, respectively, while scanning electron microscopy allows for detailed visualization of GSR particles.
The analysis of GSR aims to determine whether the residue originated directly from a firearm discharge or through secondary transfer, addressing questions about the presence, quantity, and timing of these residues. Overall, GSR serves as a crucial piece of evidence in criminal investigations involving firearms.
On this Page
Subject Terms
Gunshot residue as evidence
DEFINITION: Burned, partially burned, and unburned powder and primer that are released as a firearm is discharged.
SIGNIFICANCE: When a firearm is discharged, gunshot residue deposits on the target and the shooter, so the presence of such residue is strong evidence that a firearm has been discharged. Forensic scientists can also estimate the distance between the shooter and target at the time a firearm was discharged by the pattern and intensity of gunshot residue.
Different types of firearms use different types of ammunition, but the basic ammunition cartridge used in handguns and revolvers contains a primer, a powder, and a bullet, all enclosed in a metal casing. The primer is an explosive mixture that is commonly composed of three chemical compounds: lead styphnate, barium nitrate, and antimony sulfide. The primer is used to initiate the ignition of the powder, which is commonly a form of smokeless powder. The powder typically contains at least nitrocellulose; some powders also contain nitroglycerin, whereas others contain nitrocellulose, nitroglycerin, and nitroguanidine. As the powder burns, pressure is applied to the bullet, forcing the bullet through the barrel of the firearm and releasing the bullet at high velocity.
As the bullet is discharged from the firearm, the buildup of heat and pressure results in the release of vapors and particulates that constitute gunshot residue (GSR). GSR is composed mainly of burned and unburned particles from the primer and powder that deposit on the shooter and the target or on objects in the path between the shooter and the target. As the distance between the firearm and the target increases, the deposition of GSR is less concentrated; this fact allows forensic scientists to estimate firing distance based on the pattern of GSR.
The study of evidence from a shooting incident often involves the microscopic examination of samples, followed by color tests. The powder particles in GSR have characteristic shapes that can be determined through microscopic examination. Color tests determine the presence of GSR based on color changes that occur when areagent is added to evidence containing GSR. The modified Griess test determines the presence of nitrite compounds in the powder—the reaction between the reagent and any nitrite compounds results in an orange color. The sodium rhodizonate test determines the presence of lead from the primer—the reagent turns pink in the presence of lead. The addition of hydrochloric acid causes the pink color to change to violet, confirming the presence of lead. The primary purpose of these and other tests is to determine if a GSR trace sample is the direct result of a firearm discharge or the indirect result of a secondary transfer. With this in mind, GSR trace analysis is meant to answer questions like whether GSR target components are present, how many target components are present, where and when these components are detected, and in what quantity they are found.
Forensic scientists also use scanning electron microscopy to visualize GSR evidence. The scanning electron microscope (SEM) offers very high magnifications that allow the identification of GSR particles based on their characteristic shapes. Additionally, the SEM can be coupled to an energy-dispersive spectrometer (EDS), which enables determination of the elemental composition of the particles. The presence of lead, antimony, and barium from the primer is considered to be characteristic of GSR.
Bibliography
Bell, Suzanne. Forensic Chemistry. Pearson Prentice Hall, 2006.
"Fast Screening of Gunshot Residue Aims to Modernize Practice." National Institute of Justice, 1 Dec. 2022, nij.ojp.gov/topics/articles/fast-screening-gunshot-residue-aims-modernize-practice. Accessed 15 Aug. 2024.
Houck, et al. Fundamentals of Forensic Science. Elsevier Academic Press, 2006.
Hueske, Edward E. Practical Analysis and Reconstruction of Shooting Incidents. 2nd ed., CRC Press, 2016.
Redouté Minzière, Virginie, et al. "The Relevance of Gunshot Residues in Forensic Science." WIREs Forensic Science, vol. 5, no. 1, 23 Aug. 2022, doi.org/10.1002/wfs2.1472. Accessed 15 Aug. 2024.
Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 11th ed., Pearson, 2015.
Saferstein, Richard. Forensic Science: From the Crime Scene to the Crime Lab. 3rd ed., Pearson, 2016.