Ballistics and Toolmark Review Notes

Ballistics Overview

  • Internal Ballistics:
    • Deals with forces acting on a bullet from primer ignition to muzzle exit.
    • Influenced by:
      • Primer.
      • Powder charge.
      • Barrel condition and twist rate.
      • Bullet crimping.
      • Bullet length optimization for chamber.
  • External Ballistics:
    • Study of forces acting on bullets from muzzle exit to target impact.
    • Factors include:
      • Bullet velocity, shape, and weight.
      • Air pressure, temperature, and humidity.
      • Elevation and shot angle.
  • Terminal Ballistics:
    • Study of bullet behavior upon target impact.

Firearm Types

  • Handguns (Pistols):
    • Designed for one-handed use.
  • Long Guns:
    • Rifles or shotguns, designed for shoulder firing.
  • Shotguns:
    • Distinguished by ammunition (pellets vs. bullets) and barrels.
    • Shorter range and thinner barrels compared to rifles.

Barrel Manufacturing and Characteristics

  • Manufacturing:
    • Gun barrels made from drilled solid steel bars.
    • Drill marks on the inner surface are unique to each barrel.
  • Class Characteristics:
    • Caliber.
    • Number of lands and grooves.
    • Direction and twist of lands and grooves.
  • Individual Characteristics:
    • Striations (unique markings).
  • Changes to Barrel Characteristics:
    • Usage (firing).
    • Corrosion.

Rifling

  • Spiral grooves inside the barrel that spin the projectile.

Cartridge Components

  • Case.
  • Primer.
  • Powder.
  • Projectile(s).
  • Shotshells include a wad.

Bullet and Shell Casing Markings

  • Class Characteristics:
    • Number and widths of lands and grooves.
  • Individual Characteristics:
    • Striations.

NIBIN

  • National Integrated Ballistic Information Network (NIBIN):
    • National database for crime labs to upload digital images of bullets and shell casings.
    • Compares images to link crimes or recovered firearms.

Gunshot Residue (GSR)

  • Definition: Powder released when a bullet is fired.
  • Limitations as Evidence:
    • Indicates firearm use and shooting distance but not specific weapon.
  • Detection/Testing:
    • Swabbing.
    • Modified Greiss test.
    • SEM-EDS (Scanning Electron Microscopy - Energy Dispersive X-ray Spectroscopy).
  • Inaccurate Tests:
    • Due to contamination, degradation, and similar materials testing positive.
  • Elemental Composition of Gunpowder:
    • Barium.
    • Lead.
    • Antimony.

Firearm Evidence Collection

  • Safety prioritization.
  • Photograph firearm in original location.
  • Sketch and document the area.
  • Record firearm details: serial number, make, model, caliber.
  • Wear gloves to prevent contamination.
  • Securely package the firearm.
  • Collect related evidence (bullets, GSR).
  • Maintain chain of custody.

Tire and Footprint Evidence

  • Soft Earth:
    • Casting with dental stone for a three-dimensional cast.
  • Hard Surface:
    • Documentation (photographs).
    • Lifting methods (adhesive lifting).
    • Casting.

Toolmark Identification and Analysis

  • Comparison of class and individual characteristics to known tools.
  • Microscopic examination: size, shape, type of tool (class characteristics); imperfections, striations, patterns, microcracks (individual characteristics).
  • Collection: photographing, microscoping, casting.
  • Types of Marks:
    • Impression marks: tool pressed into a surface.
    • Striated marks: tool moved across a surface.
    • Combination marks: both impressed and striated.

Transition from Class to Individual Identification

  • Class Characteristics: Result from manufacturing of shoes, tires, and tools.
  • Individual Characteristics: Imperfections causing slight differences.

Firing Sequence

  1. Trigger pull releases hammer/firing pin.
  2. Firing pin strikes cartridge primer.
  3. Explosion ignites gunpowder.
  4. High-pressure gas forces bullet down the barrel.
  5. Bullet exits the muzzle.
  6. Gun mechanism extracts and ejects the empty cartridge case.

Factors Preventing Striation Matching

  • Grit and rust.
  • Bullet deformation and distortion after impact.
  • Fading striations.

Determining Angle and Height

  • Angle of Impact:
    • arcsin(Width/Longitude)
  • Angle of Elevation/Depression:
    • 90 degrees + given degree, then - 180.
  • Height:
    • tan(Angle of elevation or depression) \times distance
    • (tan(Angle) \times distance + ground height) / 12