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 (although lead-free or green primers are also available). 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 or nitroguanidine, and many contain stabilizers and other additives. 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. Hence, variations in primer composition, including formulations that do not contain lead, can influence the elemental characteristics of gunshot residue and affect analytical interpretation. 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 a reagent 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 produces a red to purple color change in the presence of lead. The addition of hydrochloric acid causes the characteristic 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. However, collection and handling procedures including the choice of sampling materials and methods; environmental exposure; physical activity; persistence of gunshot residue on skin, clothing, and other surfaces; and the degradation of residue components influence the  integrity and interpretability of gunshot residue evidence.

Forensic scientists also use scanning electron microscopy (SEM) to visualize GSR evidence. The 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. In addition to inorganic particles, forensic examinations may also address organic GSR, which consists of organic compounds originating from propellants and primers. Analytical techniques such as gas chromatography–mass spectrometry are used to detect these organic components and complement elemental analysis.

Bibliography

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