Class notes- Firearms, tool marks, and impression evidence PDF

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Summary

Professor James T. Spencer...

Description

4/15/19 -

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Ballistics- the science of the travel of a projectile in flight. The flight path of a bullet includes: travel down the barrel, path through the air, and path through a target - Terminal ballistics- what happens when objects hits target - Controlled expansion of burning gunpowder generates pressure (force/area) - The area is the base of the bullet (equivalent to diameter of barrel) and is constant. - The energy transmitted to the bullet will depend upon the bullet mass times force times the time interval over which the force is applied (function of barrel length) - Bullet travel through a gun barrel is characterized by increasing acceleration as the expanding gases push on it. The longer the barrel, the greater the acceleration. - Gases push bullet through the barrel; once out of barrel, gas force is no longer present, no longer accelerating - Kinetic energy (KE) = ½ MV2 Firearms - Caliber- diameter of the bore (usually in 1/100 inch or mm (.22 cal or 9mm) - Forearm identification: - Size and shape of bullet/case - Rifling match - Grooves carved on inside of the barrel - Firing pin/case impression match - Striations on bullet match - Scratches on the bullet as it moves along the barrel - Serial numbers (can be restored) - Bc people scratch it off Bullets and projectiles - Gun mechanics

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Bullets - Ideal bullet- long, heavy needle, but it would go right through the target without dispersing any of its energy - Spheres would be retarded the greatest and release more energy, but might not get to the target. - A compromise shape is a parabolic curve with love frontal area and wind-splitting shape. - Best bullet composition is lead (Pb) which is high density and cheap - Soft metal with low melting point → seals the bullet to the barrel, so no gas can get through so that all the energy is pushing the bullet - Needle like to cut through wind, travel fast, high accuracy - Sphere like shape has almost no accuracy but very powerful because it dumps energy into the target - Like a small cannon - Small entrance wound, HUGE exit wound, if any (size of the back) - Tumbling dictates the injury pattern (“terminal ballistics”) - A short, high velocity bullet begins tumbling (ricochet/bounce) more rapidly in tissue - A longer, heavier bullet might have more KE at a longer range when it hits the target … Bullet injury - Laceration and crushing- low velocity bullets, as in handguns, do virtually all their damage via CRUSHING - Temporary cavitation - Significant with projectiles travelling in excess of 1000 fps. A “permanent” vacity is caused by the path of the bullet itself, causing the wound cavity to be stretched outward - Pushes organs out of waves - Most of the damage - Shock waves - Pressure waves can cause damage - Bone beveling - Can ricochet, break through, bounce around (skull) - Entrance and exit wounds - Entrance is small - But exit: bon- exit wound is much larger → bone gets beveled

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Forming rifling - One modern method uses broach cutters to cut all the grooves simultaneously by forcing cutter down barrel and rotating the cutter

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Rifling - Barrels have: - Grooves and lands - Striations (small lines)- microscopic imperfections from rifling tools or chips of steel from branch cutter - No 2 rifle barrels, even those made in succession, will have identical striation markings Shotgun - Small pellets - Used for short/close range - Don’t really have to aim - Sends shot out in different directions Comparison microscope - Important tool for firearms examiner - Two compound microscopes combined in one unit - Can view 2 things through one microscope Bullet comparison - Obtain sample unit - Fire similar bullet from suspect gun (into water or cotton-filled tank) - Compare striations, grooves/lands, twist direction and angles between test and sample bullets - If scratches line up, then match - No two bullets will be 100% the same Cartridge cases - Pulling trigger releases the firing pin that then strikes the primer (primary explosive ignites the secondary explosive) and ignites the powder - The bullet case is propelled backwards (as the bullet moves forward- Newton’s law) and is imprinted by the breechblock (back of barrel). - Breechblock has striations that mark cases - Firing pin impression left on back of case - Spent case extractors also leave marks identifiable marks Gunpowder residues - The firing pin of a forearms strikes the primer of a cartridge that ignites sending a flame into the cartridge case. Gunpowder in the cartridge case burns, causing it to change from a solid material to a gas creating pressure within the cartridge which forces the bullet down the barrel. When the bullet exits the muzzle, pressure behind it blows … - Modern smokeless gunpowder, and black powder, contains nitrate compounds - Can look at shape of burn/residue - Firing a weapon does not fully consume all the propellant (gunpowder) - Unburned and partially burned propellant with smoke are emitted - Can determine firing distance - Chemical analysis of residues

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- Can see GSR on victim or suspect’s hand Serial number restoration - Many metal manufactured items have serial numbers stamped in them for Id - When stamped, metal crystal structure distorred/strained into the metal block. - Apply etching agent to block and the strained area will etch more rapidlyshowing original numbers. Mural on HBC: Sacco and Vanzetti - April 15, 1920- two men attacked, shot, and killed 2 security guards in Braintree Mass, escaping with $16,000 in payroll - Eyewitness said the men “looked Italian” - Found many 0.32 bullet shells made my Peters, Winchester, and Remington - Later, 2 men arrested - Sacco and Vanzetti- Italian immigrants - S&V had a 0.32 Colt revolver and 23 bullets made by Peters, Winchester, and Remington - Vanzetti also had a .32 revolver with shotgun cases identical to one found at a failed payroll attempt 4 mo. earlier - S&V were “anarchists” - V first convicted of first payroll attempt (S not charged) - Trial then 1921 for both on double murder charges- their defense sought support from many left-wing groups as a legal attach on their politics. Started the S&V Defense fund - Long trial: 59 prosecution and 99 defense witnesses (including many “quack” experts that have just seen a gun before) - Lots of evidence: much discounted and not valid but also some good evidence - Kept changing their story- sketchy - Damning evidence was that the bullets were of obsolete manufacture - Convicted and sentenced to death- executed in 1927 - Maintained their innocence - Ballistics experts used compound microscopic to compare samples -- all came to the conclusion that came from S’s gun Tool marks - Individualized marks (ex. striations) - Typical features (class characteristics) - Accidental of identifying features (individual characteristics) - Cuts, tears, gouges, or other wear marks to individualize to tool - Ex. scratches on door hinge to try to get it open - Impression evidence - Footprints - Make, model, and size of shoe - Ware patterns of show - Bitemarks (ted bundy) - Dental impression of gum, food, victims, etc - Ted bundy- convicted serial killer

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Picked up for murder two sorority women at FSU using fake name and ID - One victim had bite mark on buttocks - Casting of Bundy’s teeth taken (forcibly). Found to overlay (match) exactly the bite wound. - Defended himself but convicted and sentenced to death - Before execution admitted to more than 40 other murders (but gave no info or details) Other impression evidence - Plastic liner of fake police badge impressed on wallet...