Ballistics
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How does external ballistics help as forensic evidence?
Bullet travels in a straight line. But due to air resistance & gravity, it will gradually fall to earth if it doesn’t hit anything. Using a laser beam/string, we can find the hole in the wall (made by the bullet) and extrapolate its path. Exception of travelling straight line: bullet hitting a hard object & bouncing off (ricochet)
Describe the basic mechanics of firing a gun. (4)
1) Place an explosive and projectile in a metal tube 2) Light the fuse; fuse burns down to the explosive, causing an explosion 3) Explosion releases a huge amount of chemical energy. Causes rise in temperature & generates gas 4) Chemical energy transferred to projectile; bullet shoots down the barrel and leaves the gun
Why do improved materials make stronger barrels?
Can withstand higher temperature & pressure = more energy
What is a propellant?
an explosive, usually black powder, nitrocellulose or nitroglycerine
What is a primer?
shock sensitive compounds that ignite the propellant. Explodes when subjected to a mechanical shock. Usually Lead Styphnate
What happens to each part of the bullet when a shot is fired? (3)
Bullet: shot towards the target Bullet case: ejected from the breech Propellant: gas & particles
What are 3 kinds of forensic evidences for guns?
1) firing pin impression 2) rifling 3) gun shot residue (GSR)
What is firing pin impressions? How do they act as forensic evidence?
Firing pin hits the base of the cartridge case and leaves a characteristic mark. Fire another bullet from the gun & use comparison microscopy
What is rifling? How does it act as forensic evidence?
Series of grooves cut down the barrel in a spiral pattern (Not for shot guns or homemade weapons) Characteristic scratch marks (striations) on the bullet created as it travels the barrel. Fire another bullet from the gun & use comparison microscopy
What is gun shot residue (GSR)? How does it act as forensic evidence?
Residues from explosion of primer charge. Comes out from the breech & muzzle. Deposited near the discharged weapon (hands & clothes of gunman, target if near). Usually lost or washed off within hours. Exception: if the gun is pressed against the victim: Most GSR and gas will go into the wound. Gases exiting would cause characteristic cross shape (X)
What are 2 tests for GSR?
1) Griess Test: testing for nitrite (NO2) residues in GSR. (has many false positives) 2). Sodium Rhodizonate test: test for lead, barium and antimony from the primer. Superseded by SEM-EDX to better detect microscopic gunshot particles. (Cannot be individualised)