DISTANCE OF FIRING DETERMINATION Gunpowder Residues
When a firearm is discharged, the bullet is not the only object expelled from the weapon (see Figure 1). The violent chemical reaction of the primer and accelerant results in a cloud of molten metals, partially burned gunpowder flakes, smoke, and other microscopic debris. This residue may be found on the person who dis charged the firearm, on an entrance wound of a victim, or on other surfaces. The dis charge of a firearm, particularly a revolver, can deposit residues 3 ft or more from the hand of the shooter, and interpretations about who fired the gun can be problematic. Some of these GSR may be used to make determinations about the location of the discharged firearm in relation to its surroundings and its target.
FIGURE 1 When a firearm is discharged, the projectile is not the only thing that leaves the barrel. The residues of completely or partially burnt propellant, volatilized metals from the projectile, and the casing (if any) and wadding (in the case of shotguns) are also ejected.
The patterning of GSRs on a target is indicative of the distance from the muzzle to the target (see Figure 2). The patterning and density of the GSRs will vary with the irearm and ammunition used. Therefore, the patterns must be empirically generated by discharging a questioned irearm to make a comparison with a questioned GSR pattern. The distances tested are typically contact (where the muzzle is against the target), 6, 12, 18, 24, 30, and 36 in; other distances may be tested based on the case circumstances. Contact or near-contact bullet entrance holes demonstrate severe damage to a textile or garment. Bullets that strike an object before hitting their final target tend to have uneven edges. Typically, the greater the damage to the textile in a contact gunshot, the higher the velocity of the ammunition. Firearms discharged more than about 3 ft from the target will not impart any residues other than a bullet wipe. A bullet wipe is a residue of lead, primer mate rials, carbon, and other materials from the barrel that are transferred (“wiped”) onto the outermost surface of the target by the bullet as it passes through. For the sake of clarity and standardization, the questioned weapon is discharged onto a 1 ft by 1 ft piece of white cloth. If the irearm is not recovered as evidence, then the range of distance estimates will be greater (1–3 ft, e.g., instead of 1.5 2.5 ft) to relect the uncertainty. Additionally, many circumstances may affect the distance estimate, including position of the shooter and target, weather, and intervening items. To make the comparisons, the examiner often must visualize the GSR pattern on the target by some means. Infrared photography may reveal the residue pattern when the clothing is dark or heavily patterned (see Figure 3). The irst test that should be used to reveal residues is the Modified Griess Test. In the Modified Griess Test, a piece of desensitized photographic paper is treated with a mixture of sulfanilic acid in distilled water and alpha-naphthol in methanol. Photographic paper is desensitized by exposing the paper to a hypo solution, desensitizing it to light but making it reactive to nitrite residues. The evidence is placed target side down on the photographic paper and pressed with a steam iron filled with a dilute acetic acid solution. The resulting residues appear as orange dots on the photographic paper. This variant of the Modified Griess Test was developed and published by Scott Doyle of the Kentucky State Police Crime Laboratory, who has enjoyed great success with it.
FIGURE 2 Gunshot residue materials expand over distance and can leave a transfer pattern on any objects between the barrel and the target. When a firearm with similar ammunition is test fired, a range of patterns can be established and compared with the crime scene pattern. This leads to an estimate of the muzzle-to-target distance. This pattern was made by a 2-in muzzle-to-target distance, Federal Ammunition 0.357 Magnum, from a Smith & Wesson Magnum Revolver with a 6-in barrel.
FIGURE 3 When a shooting victim’s clothing is dark or heavily patterned, it may be difficult to see the pattern of gunshot residues (GSRs) (a). Infrared photography may reveal the GSR pattern (b).
The last chemical treatment used to visualize GSR is to spray sodium rhodizonate on the surface and then treat that area with a series of acid sprays. The residues turn pink and then purplish-blue and are easily seen. When a scene or body is examined for GSR, it is necessary to remember that lead residues may look like GSR. Lead residues may be found up to 30 ft from the muzzle and are present on the opposite side of a penetrated target.
