Forensic Ballistics: Firearm Discharge Residue Analysis

Unveiling the Clues: Firearm Discharge Residue Analysis

Firearm Discharge Residue and its Origins

When a firearm is discharged, it leaves behind a trail of evidence in the form of Gun Shot Residue (GSR). This residue is composed of burned and unburned particles from the propellant and primer. In some cases, especially with revolvers, these residues may be blown through the cylinder, leading to the potential transfer onto the shooter's hands. Employing cutting-edge technology, detecting such residues becomes a crucial step in identifying the shooter.

Methods of Firer Identification

a. Walker's Test

The GSR contains burned and unburned particles of Potassium nitrite. Various methods have been in practice for some time to collect these particles. Walker devised a test known as the 'Walker test' to collect and detect nitrite in the residue using desensitized photo bromide paper.

The technique was quite involved and was conducted as follows: 

• Desensitize a sheet of photographic paper by removing the silver halides with a 'hypo' solution.
• After drying, immerse the paper in a solution of 5% 2-naphthylamine 4, 8 disulphonic acid and let it dry.
• Place a cloth wetted with 20% acetic acid under the object to be tested.
• Place the photographic paper on top and press with a hot iron. This step softens the gel coating of the photographic paper, enabling it to pick up any GSR particles.
• Any bright red spots that appear are diazo compounds indicating the presence of nitrites.

Despite its complex nature, the test was highly efficient in picking up partially burnt propellant particles from clothing, enabling a range of firing estimations. The hand washings of the suspected firer are obtained by thoroughly washing their hands with warm water and subjecting them to the above tests.

b. Paraffin Test / Dermal Nitrate Test 

Another spot test called the 'Paraffin test' was developed to detect dermal nitrate on the hand of the firer. GSR particles are collected by applying melted paraffin wax to a subject’s hand. After removing the wax cast, a reagent containing diphenylamine and Sulphuric Acid is applied to the cast. The development of a blue colour indicates the presence of GSR.

c. Collection of Gunshot Residue on the Hands by Swabbing

GSR from the suspect's or victim's hands should be collected using cotton swabs wetted with distilled water or 0.1M HCl. Collect five different swabs from both hands and label them as follows:

• Swab on the right-hand web: A
• Swab on the right-hand palm: B
• Swab on the left-hand web: C
• Swab on the left-hand palm: D
• Collect the fifth swab from the wrist on any hand as a control: E.

Analyzing the swabs helps determine the hand used for firing. The absence of GSR on the wrist serves as a control.

d. Scanning Electron Microscope- Energy Dispersive X-ray Spectroscopy (SEM-EDX)

Modern analysis techniques include SEM-EDX. A specialized GSR collection kit is used to gather suspected gunshot particles from the shooter's hand. These particles are then analyzed using SEM-EDX. This method identifies the presence of chemical elements such as lead, antimony, and barium in the residue, offering a more precise means of identification.

 Conclusion

 Firearm discharge residue analysis is a potent tool in forensic investigation. By identifying and analyzing GSR patterns on hands and surfaces, investigators can pinpoint the firer, determine the firearm's direction, and even estimate the range of firing. In the intricate realm of criminal investigations, the study of firearm discharge residue unveils hidden clues, contributing to the puzzle-solving process and ensuring justice is served.