Firearms Evidence: Forensic Analysis and Handling

Firearms Evidence

Introduction to Firearms Evidence

Firearms evidence is a crucial aspect of forensic science, particularly in the investigation of violent crimes. It encompasses the examination of firearms, ammunition, discharge residues, and related materials to establish connections between weapons, suspects, and crime scenes.

• Definition: Firearms evidence includes physical items such as firearms, cartridge components (fired and unfired), gunshot residue on targets, and discharge residues on the hands of individuals who have fired or handled a discharged firearm.

• Applications: Commonly encountered in crimes against persons (e.g., homicide, assault, robbery) and property crimes (e.g., burglary, rape, narcotics violations).

• Types of Firearms: The most common firearms involved in criminal cases are handguns (revolvers and semiautomatic pistols), but long arms (shotguns or rifles) may also be involved.

Firearm Types and Components

Handguns

• Revolver: Example: Smith and Wesson .38 caliber, typically holds multiple rounds in a rotating cylinder.

• Semi-Automatic Pistol: Example: .380 Auto, uses a magazine to feed ammunition and automatically cycles after each shot.

Long Arms

• Shotguns and Rifles: Used less frequently in crimes but important in forensic investigations.

Ammunition and Cartridge Components

Cartridge Structure

A cartridge is the complete unit of ammunition, consisting of several components:

• Case: The container that holds all other components together.

• Primer: The ignition component, located either in the rim (rimfire) or center (centerfire) of the case.

• Propellant (Gunpowder): The chemical mixture that burns to produce gas, propelling the bullet.

• Projectile (Bullet): The part that is expelled from the barrel toward the target.

Types of Cartridges

• Rimfire Cartridges: Priming compound is distributed around the rim of the case. Common in small-caliber ammunition.

• Centerfire Cartridges: Priming compound is located in a central primer. May have rimmed, rimless, or semi-rimmed cases.

Cartridge Case Shapes

• Straight: Most rimfire and many centerfire cartridges.

• Bottlenecked: Common in centerfire rifle cartridges, designed for higher velocity.

• Tapered: Largely obsolete.

Bullets and Propellants

Bullet Types

• Full Metal Jacket (FMJ): Lead core fully encased in a harder metal shell.

• Jacketed Soft Point: Exposed lead tip for controlled expansion.

• Solid: Entirely made of a single material, often lead.

Propellant Grains

• Shape: The shape of the propellant grain (e.g., tubular, flake, ball) influences the burn rate and ballistic performance.

• Application: Different shapes are used for rimfire and centerfire ammunition to optimize performance.

Firearm and Ammunition Identification

Class and Individual Characteristics

• Class Characteristics: Features shared by a group of firearms, such as caliber, number and width of lands and grooves, and direction of rifling twist.

• Individual Characteristics: Unique marks produced by the manufacturing process or wear, allowing for the identification of a specific firearm.

Caliber

• Definition: The diameter of the bore, measured in hundredths or thousandths of an inch or in millimeters.

• Example: .38 Special has a bore of approximately 0.36 inches, despite the name.

Rifling Impressions

• Lands and Grooves: Raised and recessed areas inside the barrel that impart spin to the bullet.

• Impressions: The pattern of lands and grooves is transferred to the bullet as it travels through the barrel.

Cartridge Case and Bullet Examination

Markings on Cartridge Cases

• Breechface Impressions: Unique marks left by the breechface of the firearm.

• Firing Pin Impressions: Indentation made by the firing pin striking the primer.

• Extractor and Ejector Marks: Scratches or gouges from the removal and ejection of the case.

• Headstamp: Markings on the base of the cartridge case identifying the manufacturer and caliber.

Bullet Examination

• Comparison: Bullets can be compared to known standards using a comparison microscope to match rifling impressions.

• Caliber Estimation: Even distorted bullets can be measured or compared to known standards for caliber determination.

Collection and Handling of Firearms Evidence

Safety Considerations

• Universal Safety Rule: Treat all firearms as if they are loaded. Never point a firearm at anyone; always point it in a safe direction.

• Handling: If unfamiliar or uncomfortable with a firearm, seek assistance. If a firearm is jammed or stuck, do not attempt to clear it without help.

Documentation and Collection

• Document: Record the location and condition of the firearm (loaded/unloaded, hammer position, safety on/off).

• Photograph: Take photographs of the firearm in its original location before moving it, unless there is an emergency.

• Identification: Record the make, model, and serial number on a tag attached to the firearm.

• Evidence Handling: Use gloves to avoid contamination. Package firearms securely for transport. Ammunition should be removed and packaged separately.

Special Handling for Revolvers and Autoloaders

• Revolvers: Note the position of each round in the cylinder and document which chamber was in line with the barrel.

• Autoloaders: Remove the magazine and clear the chamber before packaging.

Gunshot Residue (GSR) Analysis

Introduction to GSR

Gunshot residue consists of particles produced by the discharge of a firearm, including partially burned and unburned powder, primer compounds, and metallic particles.

• Distribution: GSR can be found on the hands of shooters, nearby surfaces, and targets.

• Persistence: GSR can be lost from the skin within hours; collection should occur as soon as possible.

GSR Collection Methods

• Swabbing: Use moistened swabs to collect residue from the hands and other surfaces.

• Adhesive Discs: Press sticky discs onto the skin to collect particles for analysis.

• Precautions: Use gloves and avoid contact with other surfaces to prevent contamination. Place paper bags over hands if immediate processing is not possible (avoid plastic bags for living subjects).

GSR Analysis Techniques

• Neutron Activation Analysis (NAA): Detects trace elements by irradiating samples with neutrons.

• Atomic Absorption (AA): Measures barium and antimony in chemical extracts.

• Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDX): Identifies and analyzes GSR particles by their morphology and elemental composition.

Metal Detection Test

• Purpose: Detects trace metals on the hands of individuals suspected of handling firearms.

• Procedure: Apply a trace metal reagent to the hand and expose to UV light; photograph any color patterns that develop.

Distance Determination and Target Residue

Distance Determination

• Purpose: Establishes the distance between the firearm muzzle and the target at the time of discharge.

• Method: Analyze the pattern and amount of GSR on the target; photograph and sketch the residue pattern for documentation.

Collection of Target Residue

• Garments: Package each item separately to preserve residue patterns.

• Other Objects: Photograph and protect the residue pattern with a cardboard tent before collection.

Summary Table: Firearms Evidence Types and Collection Methods

Evidence Type	Collection Method	Key Considerations
Firearm	Document, photograph, unload, tag with serial number, package securely	Always treat as loaded; avoid contamination
Cartridge Case	Collect with gloves, package individually	Preserve markings (breechface, firing pin, extractor/ejector)
Bullet	Recover with forceps, package separately	Avoid altering striations or impressions
GSR (hands, clothing)	Swabs or adhesive discs, package promptly	Collect as soon as possible; avoid washing/rubbing
Target Residue	Photograph, package item separately	Preserve pattern for distance determination

Conclusion

Proper collection, documentation, and analysis of firearms evidence are essential for forensic investigations. Understanding the types of evidence, their forensic significance, and the correct handling procedures ensures the integrity of the evidence and the reliability of forensic conclusions.