Firearms Evidence Notes

Firearms Evidence

Handgun

Rifle

Shotgun

Terminology

  • Pistol: A handgun, commonly semi-automatic, uses a magazine and ejects fired cartridge cases automatically.
  • Revolver: A handgun with a rotating cylinder to hold cartridges; cartridge cases are not automatically ejected and must be removed manually.
  • Shotgun: A shoulder-fired firearm with a smooth barrel.
  • Rifle: A shoulder-fired firearm with a rifled bore.
  • Ballistics: The scientific study of the motion of projectiles.
  • Bullet: The projectile expelled from a rifled firearm.
  • Cartridge: A complete, unfired round of ammunition, including the casing, projectile, primer, and powder.
  • Firing Pin: The component of a firearm that contacts the ammunition.
  • Groove: The interior portion of a rifled barrel cut to form the rifling.
  • Land: The interior portion of a rifled barrel between two grooves.
  • Rifling: Grooves in the interior of a firearm barrel that impart spin to a bullet.
  • Shotgun Shell: A complete, unfired round for a shotgun, including casing, projectile, wadding, primer, and powder; also called a slug.
  • Wadding: Paper or plastic in a shotgun shell that forms a seal between the powder and the shot.
  • Headstamp: Markings on the bottom of a cartridge case indicating the manufacturer and caliber.

Headstamp Example

  • Example shown: R.W.S 85

Casing Comparison

  • Comparison microscopes are used to analyze similarities between casings.

Handgun Nomenclature

  • Key parts:
    • Sights
    • Slide
    • Slide Release
    • Ejection Port
    • Barrel
    • Takedown pin/lever
    • Frame
    • Hammer
    • Grip
    • Trigger
    • Trigger Guard

Ammunition Components

  • Components:
    • Projectile(s)
    • Case
    • Wad
    • Gunpowder
    • Primer
Glossary of Ammunition Components
  • Case: The container holding all other ammunition components, typically made of brass, steel, copper, paper, or plastic.
  • Gunpowder: A chemical mixture that burns rapidly and converts to an expanding gas when ignited. Modern smokeless powder burns slowly in the open, while black powder is less stable and can be explosive.
  • Primer: An explosive chemical compound that ignites the gunpowder when struck by a firing pin. It can be located in the rim (rimfire) or center (centerfire) of the case base.
  • Projectile: The object(s) expelled from the barrel. A bullet, usually containing lead, is fired from a rifle or handgun. A slug is a solid projectile, usually lead, fired from a shotgun. Shot consists of pellets made of lead, steel, tungsten alloy, or bismuth, also fired from a shotgun.
  • Wad: A seal and/or shot container, made of paper or plastic, separating the powder from the slug or shot in a shotshell. It prevents gas escape and holds the shot together as it travels through the barrel.

Bullet Projectiles

  • Bullet weight considerations.

Trajectory

  • Elements of a Trajectory:

    • Line of Departure
    • Line of Sight
    • Baseline
    • Mid-Range Height

  • Example Trajectory Chart:

    • Distances: 50, 100, 150, 200, 250 yards.

Projectile Striations

Ballistics

  • Types of Ballistics:
    • Internal Ballistics
    • Transitional Ballistics
    • External Ballistics
    • Terminal Ballistics
Internal Ballistics
  • Also known as initial or interior ballistics.
  • Deals with events occurring within about two milliseconds from firing pin contact to bullet exit from the muzzle.
  • Involves the analysis of propulsion and flight of projectiles within the firearm.
  • Example: A bullet fired from a rifle has more energy than one fired from a shotgun; this energy difference is a subject of internal ballistics.
Transitional Ballistics
  • Also known as intermediate ballistics.
  • Acts as the mediator between internal and external ballistics.
  • Studies the behavior of projectiles from propagation until pressure equalization.
External Ballistics
  • Concerned with the flight of the bullet after it leaves the barrel.
  • Studies the bullet's condition as it approaches the target.
  • Analyzes the path of the projectile.
Terminal Ballistics
  • Deals with the impact of projectiles.
  • Helps understand wounded ballistics.
  • Projectiles can cause injury in three ways:
    • Shock waves
    • Cavitation
    • Laceration and crushing

Tissue Damage

  • Bullets produce tissue damage in three ways:
    • Laceration and Crushing: Tissue damage along the projectile's path; the diameter of crush injury is the diameter of the bullet or fragment.
    • Cavitation:
      • A permanent cavity is from the bullet's track with tissue crushing.
      • A temporary cavity is formed by radial stretching around the bullet track due to continued acceleration.
        . * Low-velocity projectiles create nearly the same permanent and temporary cavities.
      • High-velocity projectiles with bullet yaw create larger temporary cavities.
    • Shock Waves:
      • Compress the medium and travel ahead and to the sides of the bullet.
      • Low velocity: cause no profound destruction.
      • High velocity: can reach up to 200 atmospheres of pressure.
      • Bone fracture from cavitation is extremely rare.
      • The ballistic pressure wave from distant bullet impact can cause concussive-like effects in humans, causing acute neurological symptoms.