Module 5: Firearms and GSR Trace Evidence

IBIS

Integrated Ballistic Identification System

Integrated Ballistic Identification System (IBIS)

-captures high-resolution 3D images of marks left on bullets and casings by firearm's rifling, firing pin, and breech face

-compare digital images to find links between firearms and crimes

NIBIN

National Integrated Ballistic Information Network

CIBIN

Canadian Integrated Ballistics Identification Network

Firearms Searches/Database:

Link cases committed with same weapon

• Identify serial offenders using same firearms in multiple crimes

• Track firearms: e.g., recovered firearms test-fired and evidence can be entered into database to assess links to unsolved cases

Comparing a test-fired bullet to a case sample involves….

comparing and aligning features using the comparison microscope.

CMC

Congruent matching cells

KM

Known Match

KNM

Known Non-match

Gunshot residue (GSR): Formed when…

firing pin strikes primer and primer explodes and bullet exits barrel carried on pressure wave:

Gunshot residue (GSR): includes mixture of…

burnt/unburnt portions of powder, bullet casing and barrel material

Gunshot residue (GSR): Residues from primer focus of most casework, composed of…

shock-sensitive explosives that include metallic elements.

Gunshot residue (GSR): Analysis focus on…

lead (Pb), barium (Ba), antimony (Sb), and also aluminum (Al), calcium (Ca), and zinc (Zn).

Gunshot residue (GSR): When primer explodes hot enough to…

break chemical bonds and vaporize components that recondense as they cool into tiny particulates called GSR

Forensic Applications of GSR:

Identifying Suspects:, Determining Firing Distance, Reconstructing Crime Scenes:, Corroborating or Refuting Testimony, Exonerating Innocent Individuals, Differentiating Entry and Exit Wounds

GSR: Identifying Suspects:

GSR particles on hands, face, or clothing can indicate: recent discharge of a firearm, proximity to a discharging firearm, or handling after firing

GSR: Determining Firing Distance:

pattern and density of GSR around a bullet hole on a victim's clothes or other surfaces can estimate distance between firearm's muzzle and target at time of discharge. (often involves initial presumptive tests)

GSR: Reconstructing Crime Scenes:

distribution and concentration of GSR particles at scene (e.g., walls, car headliner) helps reconstruct events (e.g., shooter's position and number of shots fired)

GSR: Corroborating or Refuting Testimony:

e.g., absence of GSR if claimed weapon fired in self-defense; suggests account is inconsistent with evidence.

GSR: Exonerating Innocent Individuals:

absence of GSR on a person can support claims of non-involvement (but see time since deposition variables)

GSR: Differentiating Entry and Exit Wounds:

see firing distance above for range, but GSR normally found in higher concentration at entry wound vs exit

Presumptive Testing:Modified Griess test and Walker test common presumptive tests for…

nitrites found in GSR ; nitrites are by-products of gunpowder combustion but not confirmatory if present

Walker test :

Common to determine if GSR present on clothing (turns red).

Modified Griess test

common to determine gun to target distance.

Sodium rhodizonate test

detects Pb & Ba -turns red/purple color when lead is present -reddish-brown when exposed to barium -not for precise gun to target distance -often used determine if hole consistent with passage of a bullet

Scanning Electron Microscopy (SEM)

Analysis of GSR involves physical and chemical methods, often using SEM

SEM primary application is for small particulates such as….

gunshot residue

SEM: Technique even more powerful when SEM combined with an….

energy-dispersive X-ray spectrometer (EDS)

SEM/EDS combination resolves particle structure smaller than…

1 µm in size while generating spectra revealing the elemental composition of the object.

GSR – Scanning Electron Microscopy (SEM) • Samples collected using an…

SEM stub from hands, clothing, or another surface; particulates stick to stub

SEM utilizes electrons to create….

images where electron beam scans surface of stub, software tags particulates consistent with shape (smooth, roughly spherical) and size of GSR (0.5–5.0 μm in diameter)

Challenges linking gunshot residue to a specific source: If ammunition used specifically tagged in some way by special elements, it is possible to…..

link cartridge used to produce GSR

Challenges linking gunshot residue to a specific source: Inference about source of gunshot residue can be based on…

examination of the particles found on a suspect and population of particles found on victim, in firearm or in cartridge case

Ammunition variability:

Even within same brand, different batches of ammunition may have slight chemical variations (e.g., test fire vs unknown)

Challenges linking gunshot residue to a specific source: Different types of ammunition (e.g., lead-free or .22 caliber) may not produce….

conventional lead, barium, and antimony particles, making them harder to trace

Challenges linking gunshot residue to a specific source: Name 3

Ammunition variability, If ammunition used specifically tagged in some way by special elements, it is possible to link cartridge used to produce GSR, What does relative amount of GSR on suspect mean (E.g., 5 particles vs 500 particles) in context of changes with time and 2ndary transfer?