A powdery residue is created when a firearm is discharged. The residue can be helpful in forensically linking a suspect to the scene of the gunshot.
A bullet is propelled out of a gun or rifle at very high speed when the gunpowder in the barrel of the firearm is ignited. The ignition converts the solid gunpowder to a gas, which creates the pressure that propels the bullet outward. As the bullet emerges from the gun barrel, the gunshot residue is also propelled outward at high speed. The residue fans outward, forming a cone shape. Nearer the gun barrel, the residue is very concentrated. With increasing distance from the gun barrel, the residue becomes progressively less concentrated.
Gunshot residue can travel out from the gun to distances of 3–5 feet (0.3–1.5 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 involved in the firing of the gun. Close to the gun barrel, the residue deposits more heavily on surfaces like skin and clothing, to the point of being visible as a dark stain. Detection of a significant amount of residue, therefore, is a powerful piece of forensic evidence that the particular person was very near to, even holding, the gun when it discharged.
Residue can also be deposited on skin or clothing when a person briefly contacts a victim. In this case, only a light application of residue will be detectable.
This pattern can be critical in exonerating a suspect. For example, in 2005 actor Robert Blake, star of the 1970s television hit show Baretta, was found not guilty of the shooting murder of his wife. One of the important pieces of evidence was the absence of all but a trace of gunshot residue. It was successfully argued that the residue could have resulted from his handling of his wife's body or touching the interior of the car in which she was found.
Gunshot residue consists of tiny balls, flakes, or discs of the expelled gunpowder. The various shapes can be revealed by microscopic examination. The shape of the residue is governed in part by the composition of the gunpowder. Determining the type of gunpowder in the suspect firearm is a helpful step in a forensic investigation. In particular, the use of a scanning electron microscope equipped with an elemental analyzer permits the determination of the elemental composition of the residue.
Depending on the gunpowder, gunshot residue can contain lead residue, although lead-free ammunition is becoming more popular. As well, modern smokeless gunpowder residue can contain traces of nitrate, charcoal, and sulfur. Nitrocellulose and nitroglycerin may also be present.
see also Ballistics; Firearms; Scanning electron microscopy.