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.

Test patterns from various distances

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

Sticky stub used to collect GSR

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!

Back to Step #2

Continue to Step #4

Report abuse