Estimating Distance
Step #3: Estimating Distance through GSR
Now that you know how toolmarks can help firearm investigators distinguish between class and individual characteristics, you are ready to learn how they use the laws of physics to determine the distance the shooter was from the target.
Gun Shot Residue
When a firearm is discharged, un-burned and partially burned particles of gunpowder, in addition to smoke, are propelled out of the barrel along with the bullet towards the target.
Gunshot residues may also move backwards towards the shooter's hands.
Muzzle/Cylinder Residues
- Un-/Partially-burned powder
- Macroscopic – 10X-40X
- Look for on target
- Used to determine muzzle to target distance
Primer Residues
- Primer Residues
- Microscopic 4000X
- Look for on solid objects: usually shooter’s hands
- Traditional GSR (gunshot residue)
Muzzle to Target Distance
The process of determining the distance between the firearm and the target is usually based on the distribution of powder patterns or the spread of a shot pattern.
- The first step is to create Test Patterns with known distances.
- Those test patterns are then compared to the evidence pattern
- Shot patterns allow for longer range estimation
The dark circle around the bullet hole is knows as the bullet wipe. The ring is formed by and contains bullet lubricant, byproducts of propellant, traces of bullet metal, and residue in the gun barrel from previous use.
Investigators may use Infared Photography or chemical tests to find gun shot deposits on a target. The Griese Test is used to detect nitrates or nitrites. The Sodium Rhodizinate is used to detect lead.
Primer Residue (GSR)
There are several tests used determine the presence of gunshot residue, but it first must be collected from a suspect. One method is to use swabs which resemble Q-tips and another technique is to use a "sticky stub". GSR is the consistency of flour and typically only stays on the hands of a living person for 4–6 hours. Wiping the hands on anything, even putting them in and out of pockets can transfer GSR off the hands. Victims don't always get GSR on them; even suicide victims can test negative for GSR.
GSR Tests
Atomic Absorption Spectrophotometry (AAS)
- Looks for presence of barium (Ba), antimony (Sb), and lead (Pb)
- Is not specific for GSR
Scanning Electron Microscopy (SEM)
- Identifies particles which are presently considered unique to GSR
- Involves both visual and chemical identification
Organic Methods
- Being used with “leadless” ammunition; looks for nonmetal components of GSR
- Most techniques are still in development
In this step, you learned how investigators can use gunshot residue patterns to estimate the distance between the muzzle and the target. You also learned that there are several tests used to detect primer residue, or GSR. Detection of GSR on a suspect's clothes or hands would be strong evidence that they fired a weapon within the last 4 hours.
Now you are ready to move on to calculating firearm trajectories!