#14 - firearms cont.

Chapter Overview

  • Discharge of Firearm

    • Involves combustion aspects.

    • Involves shock-sensitive explosive primer and propellant powder that deflagrates.

  • Conversion of Energy

    • Chemical potential energy to kinetic energy.

    • Loss of energy to heat and flash.

  • Evidence Produced

    • Discharge produces chemical and physical evidence.

    • Key forensic questions include determination of whether an individual fired a weapon and distance from barrel to target.

Energy Dynamics in Firearm Function

  • Heat (Q) Loss

    • Q = mCAT of gases.

  • Energy Transition

    • Chemical energy to kinetic energy equation: 1/2 mv².

    • Include miscellaneous losses (light, incomplete combustion, barrel friction).

Operation of Firearms

Small Arms: Revolver

  • Action Mechanism

    • Firing pin impact ignites the primer.

    • Components: Cylinder, firing pin, barrel, hammer, rifling, trigger, main spring.

Small Arms: Semi-Automatic Pistol

  • Action Mechanism

    • Components: Cartridge, barrel, chamber, slide, hammer, recoil spring, magazine, trigger.

    • Gas pressure is used to eject the spent casing.

Bullet Trajectory and Spin

  • Imparting Spin

    • Heat assists projectile grip on rifled surface; ensures a stable trajectory upon leaving the barrel.

Cartridge Casings

  • Revolvers

    • Casings remain until manually ejected.

  • Semi-Automatic Pistols

    • Casings ejected automatically by gas pressure.

Firearm Calibration and Ammunition

Caliber and Gauge

  • Caliber

    • Nominal barrel diameter, described in inches (e.g., 0.22, 0.38, 0.45) or metric (9mm).

  • Gauge

    • Relates to the number of pellets per pound for shotguns (e.g. 12-gauge vs. 20-gauge).

Types of Cartridges and Primer Arrangements

  • Rimfire and Centerfire Cartridges

    • Rimfire: Smaller caliber (e.g., 0.22).

    • Centerfire: Detonated at primer center.

Chemical Constituents of Firearms

  • Primers

    • Shock-sensitive explosives.

  • Residues

    • Key form of evidence; deposited on shooter and near surroundings.

  • Chemical Evidence

    • Each firing event is unique; residues vary.

Key Chemical Compounds in Firearms

Common Compounds

  • Nitrocellulose (NC)

    • Energetic, gas source.

  • Nitroglycerin

    • Energetic, gas source.

  • Diphenylamine (DPA)

    • Stabilizer.

Additional Compounds

  • Stabilizers

    • Methyl centralite (MC), Ethyl centralite (EC).

  • Flash Suppressors and Explosives

    • 2,4 Dinitrotoluene (24DNT), Lead styphnate, Barium nitrate.

Propellant Chemistry

Historical Insight

  • Black Powder

    • Original propellant with smoke; replaced by smokeless alternatives.

  • Smokeless Powder

    • Nitrocellulose as primary energetic propellant.

Burn Rate Control

  • Parameters of Burn Rates

    • Progressive, neutral, and regressive burn types control for optimal propulsion.

Energy Transitions in Firearms

  • Chemical Energy to Kinetic Energy

    • Efficiency impacts muzzle velocity; high-pressure conditions lead to specific particle formation.

Firearm Residues and Forensic Analysis

Residue Analysis

  • Firearm discharge residue (FDR) includes all chemical residues.

  • Terminology

    • OGSR: Organic discharge residue containing unburned powder.

    • Key elements: Barium (Ba), Antimony (Sb), Lead (Pb).

Range Estimation

  • Estimation of distance critical in investigations; various techniques including color tests and newer instrumentation.

GSR Formation and Characteristics

  • GSR Formation Process

    • High temperatures and pressures produce smooth spheroid particulates (0.5-5.0 µm) due to vaporization and re-condensation.

Particle Categorization

  • Typical Combinations

    • Characterization of particles associated with lead and lead-free/non-toxic primers.

Combustion Aspects

  • Detailed discussion on deflagration versus detonation and their forensic implications.