Forensic Ballistics: Interior and Exterior Ballistics

• Forensic ballistics is the science of analyzing firearm-related evidence to aid in criminal investigations

Interior Ballistics

• Focuses on the processes occurring within a firearm from the moment the trigger is pulled until the projectile exits the barrel
• Ignition of the Propellant: When the trigger is pulled, the firing pin strikes the primer, igniting the propellant (gunpowder)
• Pressure Buildup: The burning propellant generates rapidly expanding gases, creating immense pressure inside the cartridge and barrel
• Projectile Acceleration: The pressure forces the projectile (bullet or shot) down the barrel at high speed
• Rifling Engagement: In rifled firearms, the projectile engages with the rifling (grooves) inside the barrel, causing it to spin
• Spin Stabilization: The spin imparted by the rifling stabilizes the projectile in flight, improving accuracy and range

Exterior Ballistics

• Deals with the projectile's behavior after it leaves the firearm's barrel until it reaches its target
• Initial Velocity: The speed of the projectile as it exits the barrel is a critical factor affecting its trajectory and impact
• Trajectory: The path of the projectile through the air is influenced by gravity, air resistance (drag), and the projectile's spin
• Air Resistance: Drag slows the projectile, reducing its velocity and affecting its trajectory
• Gravity: Gravity constantly pulls the projectile downwards, causing it to drop over distance
• Spin ( gyroscopic stability): The projectile's spin stabilizes it against tumbling, increasing accuracy and range
• Range: The maximum distance a projectile can travel depends on its initial velocity, ballistic coefficient, and firing angle
• Flight Time: The duration the projectile spends in flight is influenced by its velocity and trajectory

Terminal Ballistics

• Examines what happens when the projectile strikes a target
• Impact Velocity: The projectile's speed at the moment of impact affects the amount of energy transferred to the target
• Penetration: The depth to which a projectile penetrates a target depends on its velocity, mass, shape, and the target's composition
• Deformation: Projectiles can deform upon impact, changing their shape and affecting their penetration and wounding potential
• Fragmentation: Some projectiles are designed to fragment upon impact, creating multiple wound channels
• Energy Transfer: The amount of energy transferred to the target determines the extent of damage or injury
• Wound Ballistics: A subfield of terminal ballistics that studies the effects of projectiles on living tissue, including wound size, shape, and severity

Firearm Mechanisms

• Revolvers: Firearms with a rotating cylinder containing multiple chambers, each holding a cartridge
• Semiautomatic Pistols: Firearms that automatically reload a new cartridge into the chamber after each shot, but require a separate trigger pull for each shot
• Rifles: Long-barreled firearms designed to be fired from the shoulder, typically with rifled barrels for increased accuracy
• Shotguns: Long-barreled firearms designed to fire multiple projectiles (shot) or a single projectile (slug)
• Machine Guns: Fully automatic firearms that continuously fire as long as the trigger is held down

Ammunition Components

• Cartridge Case: The container that holds all the other components of ammunition
• Primer: An explosive compound that ignites the propellant when struck by the firing pin
• Propellant: The substance (gunpowder) that burns rapidly, producing gases that propel the projectile
• Projectile: The bullet or shot that is expelled from the firearm

Bullet Types

• Lead Bullets: Simple, inexpensive bullets made of lead, often used in low-velocity firearms
• Jacketed Bullets: Lead bullets encased in a jacket of harder metal (usually copper) to improve penetration and reduce lead fouling
• Hollow-Point Bullets: Bullets with a cavity in the nose designed to expand upon impact, increasing energy transfer and creating a larger wound cavity
• Frangible Bullets: Bullets designed to break apart upon impact, reducing the risk of ricochet and over-penetration

Shot Shell Components

• Shot: Small, spherical projectiles made of lead, steel, or other materials
• Wad: A plastic or paper cup that separates the shot from the propellant and helps to seal the bore
• Hull: The cartridge case of a shotshell, typically made of plastic or paper

Rifling Characteristics

• Lands: The raised portions of the rifling inside a gun barrel
• Grooves: The recessed portions of the rifling inside a gun barrel
• Twist Rate: The rate at which the rifling twists inside the barrel, expressed as the distance required for one complete rotation (e.g., 1 turn in 10 inches)

Forensic Examination of Firearms

• Firearm Identification: Determining the make, model, and caliber of a firearm
• Function Testing: Examining a firearm to ensure it is functioning correctly and safely
• Trigger Pull Analysis: Measuring the amount of force required to pull the trigger
• Muzzle-to-Target Distance Determination: Estimating the distance between the firearm and the target at the time of firing
• Serial Number Restoration: Recovering obliterated serial numbers from firearms

Microscopic Comparison of Bullets and Cartridge Cases

• Striations: Microscopic markings on bullets and cartridge cases caused by imperfections in the firearm's barrel and breech face
• Firing Pin Impressions: Marks left on the primer of a cartridge case by the firing pin
• Breech Face Marks: Marks left on the cartridge case by the breech face of the firearm
• Ejector Marks: Marks left on the cartridge case by the ejector mechanism
• Extractor Marks: Marks left on the cartridge case by the extractor mechanism

Ballistic Evidence Collection

• Documenting the Scene: Thoroughly photographing and documenting the location of all ballistic evidence
• Recovering Firearms: Carefully collecting and securing firearms, ensuring they are unloaded and properly packaged
• Collecting Bullets and Cartridge Cases: Collecting all bullets and cartridge cases found at the scene, taking care not to damage them
• Trajectory Analysis: Using lasers or rods to determine the path of projectiles
• Chain of Custody: Maintaining a detailed record of the handling and storage of all evidence

Toolmark Examination

• Toolmarks: Impressions or scratches left on a surface by a tool
• Comparison: Comparing toolmarks found at a crime scene with those produced by a suspect tool
• Impression Marks: Marks created when a tool is pressed into a surface
• Striated Marks: Marks created when a tool slides across a surface

Legal Aspects of Forensic Ballistics

• Admissibility of Evidence: Ensuring that ballistic evidence is collected, analyzed, and presented in a legally sound manner
• Expert Testimony: Providing expert testimony in court regarding ballistic evidence and its interpretation
• Daubert Standard: Meeting the Daubert Standard for scientific evidence, which requires that evidence be reliable, relevant, and based on sound scientific principles

Safety Considerations

• Handling Firearms: Always handling firearms with extreme care and following all safety rules
• Unloading Firearms: Ensuring that firearms are unloaded before examination
• Personal Protective Equipment: Wearing appropriate personal protective equipment, such as gloves and eye protection
• Evidence Handling: Properly packaging and storing ballistic evidence to prevent contamination or damage