Gunshot residue is composed of smoke resulting from combustion of propellant, metallic fragments from the bullet, unburned and partially burned gunpowder particles, and lubricants. Gunshot residue may be deposited on surfaces at a crime scene or on clothing or skin of persons present during the weapon discharge. Visualization of gunshot residue is an important task during preliminary phases of criminal investigations and can help investigators in a variety of ways.
Gunshot residue, particularly gunpowder discharge, is easily observed on light colored surfaces due to its inherent dark color. Dark surfaces, however, often obscure it. The two visualization methods chosen for this research include infrared light and alternative light sources.
Method: This presentation encompasses two studies. In the first study, the authors fired four types of ammunition (.22, .223, 9mm, .45) five times each from six inches into black, cotton tee-shirt samples. The authors later observed the samples using portable alternate light sources with 395nm, 450nm, and 530nm wavelengths and appropriate barrier filters. The authors utilized an infrared camera with IR 695 and IR 850 filters, in conjunction with 100- watt incandescent light sources, to observe the samples with infrared light.
In the second study, the authors fired .40 and 9mm ammunition five times each from distances of 3 inches, 9 inches, and eighteen inches into black, cotton tee-shirt samples in an effort to analyze qualitative differences between the two methods at various muzzle-to-target distances. The authors observed the samples using an alternate light source with a 450nm wavelength and orange barrier filters, since this was the most effective wavelength identified in the first study. The authors utilized the same equipment employed in the first study to visualize gunshot residue with infrared light. All samples in both studies were viewed with the same equipment prior to shooting. No particles were detected