FRSC 1011: Firearms, Tool Marks and Other Impressions

Overview

• Firearms: Different types including handguns and long guns.
• Gun Barrel Markings: Unique markings left on bullets by the gun's barrel.
• Bullet Examination: Analysis of bullets to link them to specific firearms.
• Gunpowder Residue: Residues left after firing a projectile.
• Primer Residue: Traces found on hands after firing a weapon.
• Collection of Firearms Evidence: Proper techniques for collecting and securing evidence.
• Serial Numbers: Methods for restoring obliterated serial numbers.
• Tool Marks and Other Impressions: Examination of impressions left by tools and footwear, as well as their collection and comparison.

Firearms Types

• Handguns:

  • Single-shot pistols: Fire one round at a time.
  • Revolvers: Multiple firing chambers in a revolving cylinder; variations include swing-out, break-top, and solid frame.
  • Semi-automatic pistols: Removable magazine, one shot per trigger pull.

• Long Guns:

  • Shotguns: Use shell ammunition with numerous projectiles; features include a choke to concentrate shot.
  • Rifles: Barrel has lands and grooves; bullets are impressed with lands and grooves when fired.

Gun Barrel Markings

• Markings on Bullets: Each gun imparts unique markings on a bullet as it passes through the barrel, making it identifiable.
• Rifling: Rifled barrels have spiral grooves that cause bullets to spin for enhanced accuracy; lands (ridges between grooves) and grooves (depressions) are key identifiers.
• Caliber Measurement: The diameter of the barrel, measured between lands, defines the caliber; characteristics remain consistent for each weapon.

Striations in Gun Examination

• Striations: Fine lines inside the barrel that are unique to each barrel due to imperfections; essential for individualization.
• Comparison of Bullets: Bullets can be compared using indicators like the number of lands, grooves, and their striation patterns.
• Comparison Microscope: Vital for side-by-side examination of evidence bullets against test-fired bullets.

Bullet Examination Process

• Unusual Marks: Comparison begins with class characteristics like number and direction of grooves; differences automatically eliminate the potential connection.

Gunpowder Residue

• Residue Emission: When fired, unburned or partially burned particles of gunpowder and smoke can be deposited on targets near the shot.
• Distance Assessment: The density and pattern of residues assist in determining the distance from which the weapon was fired.
• Testing Methods: Utilization of the Greiss test for detecting residues, where NO2 is a byproduct of firing smokeless gunpowder.

Primer Residue on Hands

• Detection Techniques: Measuring barium and antimony levels to ascertain if a person fired a weapon recently; particle morphology provides additional confirmation.

Firearm Evidence Collection

• Collection Guidelines: Hold firearms by the trigger guard or grip edge, ensure they are unloaded, and document any cartridges found.
• Bullets Handling: Wrap bullets in tissue before placing them in secure containers.

Tool Marks and Other Impressions

• Tool Marks: Impressions made when tools contact other surfaces; class and individual characteristics can provide evidence.
• Impression Handling: Photograph and preserve impressions before moving from the crime scene; use casting techniques for impressions in soft surfaces.

Recovery of Serial Numbers

• Restoration Techniques: Serial numbers can often be restored using chemical etching to reveal strains in the metal under original numbers.

Additional Considerations

• Comparison Software: Technology aids in shoe prints, tire impressions, and even bite mark comparisons.
• Points of Comparison: Confirming uniqueness is essential for linking evidence to a singular source.