Forensic Ballistics: Projectile Motion and Firearms ID

• Forensic ballistics is the study of projectile motion as it applies to law, as well as the science of identifying firearms through ammunition fired from them.

Origin of the word Ballistics

• Ballistics is derived from the Greek words "Ballo" and "ballein", meaning "to throw".
• Ballista was the name of a Roman war machine, a large catapult used to hurl stones, dead animals, and even dead people over long distances.

Ballistics

• Ballistics is the science of projectile motion and the conditions affecting that motion.
• A projectile is a metallic or non-metallic ball propelled from a firearm.
• Motion refers to the mobility or specific movement of a projectile from the time it leaves the shell, exits the gun muzzle, and until it hits its target or falls to the ground.

Types of Projectile Motion

• Direct Motion: The forward movement of a bullet or shot out of a shell due to the expansive force of gases from burning gunpowder.
• Translational Motion: The movement of a bullet after hitting a target and ricocheting without any rotation.
• Rotatory Motion: The bullet's movement passing through a rifled bore barrel firearm, twisted either to the left or the right.
• Gyroscopic Action: The stillness or steadiness of a bullet in flight.
• Key Hole Shot: The tumbling of a bullet in flight hits the target sideways.
• Yaw: The instability of a bullet after it is fired.

Branches of Ballistics

• Early ballistics study was divided into three divisions; interior/internal, exterior/external, and terminal ballistics.
• There are now currently Four Branches of Ballistics.
• Forensic Ballistics: Used to solve legal problems related to firearms and ammunition.

Interior (Internal) Ballistics

• Refers to the motion of the projectile while inside the firearm, extending from the breech to the muzzle.
• The conditions attributed to internal ballistics:
  • Firing pin hitting the primer: Trigger pull releases the hammer, pushing the firing pin to hit the cartridge base where the primer is.
  • Ignition of the priming mixture: The priming mixture (KCLO3, sulfur, and carbon) ignites upon the firing pin's strike, known as “Percussion action.” Percussio is Latin for "to beat or strike.
    • Alexander John Forsyth: The father of percussion powder and invented the priming mixture which caused the integration of the primer, bullet, gunpowder, and shell.
  • Combustion of gunpowder/powder charge/propellant: Ignition is imparted to the gunpowder via a vent/flash hole turning it into an expanded hot gas.
    • Vent (Flash hole): The hole at the bottom of the primer pocket allowing passage for the priming mixture.
  • Expansion of heated gas: Once burned, the charged powder turns into heated gas, spreading and filling the cartridge case/shell.
  • Pressure developed: Outward gas pressure from burned powder inside the shell causes the bullet to withdraw.
  • Energy generated: Energy is used as a propellant for gases; the pressure developed forces the projectile out, measured in foot-pounds.
  • Recoil of the Gun: The equal and opposite reaction of the gun against the bullet's forward movement upon explosion.
    • Jump: A portion of recoil is characterized by backward and upward movement before the bullet leaves the muzzle.
  • Velocity of the bullet inside the barrel: The bullet's relative speed per time unit while inside the barrel or at the muzzle.
  • Rotation of the bullet inside the barrel: The twist (right or left) of the bullet caused by the cylindrical grooved portion of the barrel.
  • Engraving of the cylindrical surface of the bullet: After passing through any firearm barrel (rifled or smooth bore), imperfections/scratches from the interior barrel form on the bullet's cylindrical surface "thumbprint."
    • Marks made by the barrel or any part of the firearm are considered an individual characteristic.