The gunshot residue is the unburnt or half-burnt particles discharged from a firearm. Collection of gunshot residue can be done in numerous ways.
Gunshot residue is also termed as firearm discharge residue and gunfire residue. Gunshot residue is usually formed when a weapon or a firearm is discharged. The vapors released by the primers are solidified into particles and adhere to people or objects around. GSR particles greatly vary in shape and size, and hence the examiner must collect and analyze all types of particles found so that they won’t miss out on any evidence.
The quantity and quality of the GSR particles are affected by various factors such as the type of the gun, caliber, bullet, the burn rate of the powder charge, and the environmental condition.
Sources of Gunshot Residue
The most primary source of gunshot residue is the primer and the propellant. It also consists of the fragments of the bullet, cartridge cases, and the firearm.
Primer consists of a ratio of multiple chemicals such as lead styphnate, barium nitrate, and antimony sulfide. The particle-containing all three elements are considered to be unique gunshot residue.
The force to propel out the bullet is produced by the propellant. Three major types of propellant charges are
- Black Powder– It is the oldest explosive known to man. It is an intimate mixture of the three ingredients: potassium nitrate(KNO3), charcoal, and sulfur in the approximate ratio of 75:15:10. Black powder is primarily used in muzzle loading guns and blank cartridges. This black powder produces smoke and fouling on the target.
- Smokeless Powder– The basic components of a smokeless powder are nitroglycerine and nitrocellulose. Nitrocellulose is alone used or used in combination with nitroglycerine to form a smokeless powder.
- Semi-Smokeless Powder– An intimate mixture of nitrocellulose, potassium nitrate, charcoal, and sulfur in the approximate ratio of 20:60:12:8. It develops less smoke compared to gunpowder.
Collection of Gunshot Residue
Three main places are examined for the gunshot residues. These three places are the hands of the shooter, clothing, and the area surrounding the shooter.
The shooter’s hands are the most common place that is preferred for the collection of the gunshot residue from a suspected shooter.
Collection of the gunshot residue is also done on the clothes, as they tend to preserve the GSR even if there is no longer any residue present on the shooter.
A cloud of gunshot residue will spread around in the air from the opening of the gun and land on objects around the shooter.
The standard time frame for collecting gunshot residue is three to six hours after the incident has occurred. It can last long on clothing and deceased individuals.
Methods of Collecting Gunshot Residues
Several methods are used to collect GSR. The important methods are given below:
- Dry Method of Collecting GSR
- For the collection of gunshot residue on hands, molten wax of suitable melting point is gently brushed over the hands till it acquires a minimum thickness of 1 to 2 mm. When the wax is dried and set. It is peeled off, picking up the gunshot residue present on the hands.
- A solution of cellulose acetate is applied to the site bearing the GSR. It is then peeled off upon drying. The cast picks up the gunshot residues.
- The site bearing the residue is sprayed with the collodion solution. The film is reinforced with nylon fibers. The reinforced film which picks up the powder is peeled off on drying.
- A cellophane sheet impregnated with acetic acid is pressed against the site. It picks up the lead.
- Wet Method of Collecting GSR
- A filter paper is moistened with dilute acetic acid. It is then pressed against the spot suspected to bear GSR. Collection of the gunshot residue is then done using filter paper.
- A piece of cotton cloth or cotton swab is moistened with dilute hydrochloric acid(10%) or with nitric acid (5%), and the site bearing the GSR is swabbed with this piece of cotton cloth or swab. It picks up the GSR. The swabs from various parts of the hands are collected separately.
- The hands are rinsed thoroughly in dilute nitric acid placed in a plastic bag. The solution thus obtained is freeze-dried and is ready for testing purposes.
- The residues in the barrel are collected by washing the barrel with hot distilled water. The washings obtained are tested for the constituents of the residues.
- Tape Lifting
An inherent tape of suitable width is taken, and the site is taped to collect GSR. The tape as such or extracted is used for further testing.
- Vacuum Lifting
Vacuum lifting is helpful for the collection of GSR from the clothes.
Gunshot residue is understood to be among the foremost common and most scrutinized sources of evidence examined in violent investigations.
Because of such great importance, it is necessary for investigators to be trained in the collection of gunshot residue and should have some knowledge of forensic science.