Ballistics Flashcards
Ballistics
the science of the travel of a projectile in flight
Firearm Identification
Size and shape of bullet/case
Firing pin/case impression match
Striations on bullet match
Serial numbers (can be restored)
Bullet Injury
Laceration and crushing→ low velocity bullets, as in handguns, do virtually all their damage via crushing
Cavitation→ significant with projectiles traveling in excess of 1000 fps. A “permanent” cavity is caused by the path of the bullet itself,, causing the wound cavity to be stretched outward
Shock waves→ shock waves compress the medium and travel ahead of the bullet, as well as to the sides. At high velocity, generated shock waves can reach up to 200 atmospheres of pressure
Rifling
Made by drilling spiral grooves into barrel
The faster the spin, the less likely a bullet will “yaw” or turn sideways and stumble. Thus, increasing the twist of the rifling from 1 in 7 will impart greater spin than the typical 1 in 12 spiral (one turn in 12 inches of barrel)
Bullet Travel
Bullets do not typically follow a straight line to the target. Rotational forces are in effect that keep the bullet off a straight axis of flight
Bullet Comparison
Impressions made on soft bullet by running through grooves and lands
Since the rifling is worn away irregularly by each shot, the grooves make a unique impression on the bullet
Obtain sample bullet
Fire similar bullet from suspect hun (into water, gel, or cotton filled tank) *****
Compare striations grooves/lands, twist direction and angles between test and sample bullets
Problem: each successive bullet will be a little different due to wear of barrel so matchup will not be perfect
Comparison Microscope
Important tool for firearms examiner
Two compound microscopes combined into one unit
When viewer looks through the eyepiece, a field divided into two equal parts is observed
Bullet comparisons requires reflective light
Cartridge Cases
Pulling trigger releases the firing pin that then strikes the primer and ignites the powder
The bullet case is propelled backwards (as the bullet moves forward) 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 identifiable marks
Serial Number Restoration
Many metal manufactured items have serial numbers stamped in them for ID
When stamped, metal crystal structure becomes distorted or strained in the metal black
Apply etching agent to block and the strained area will etch more rapidly→ showing original numbers
Scratch out numbers can be restored since stamping marks on metal deforms layers deep into the metal structure
Sacco and Vanzetti Verdict
1927, ballistics experts test fired bullet into cotton and used the new comparison microscope
All agreed there was no doubt that the fatal bullet come from Sacco’s gun→ change of mind usually not mentioned
Sacco and vanzetti sent to electric chair in 1927
Tool Mark Evidence
certain defects or patterns may be left on a tool when it is made or used which can be used to find matches between evidence at a crime scene and tools or objects found at a suspect’s home
Tool marks can be classified two ways:
Impressions→ as a tool hits a softer surface, the shape of the tool and imperfections in its surface may be left behind as an impression
Scratches→ as a tool moves across a surface, it may leaves ridges or striations behind (NOTE: some tool marks are a combination of both types)
Tool Impression Features to analyze
Dimensions of the impression
Ridges or striation patterns
Defects, such as nicks and chips
Pain chips or metal shards left on a tool
Analyzing Bite Marks
Bite marks are photographed with a scale
Bite marks on skin are taken over repeated intervals
Casts of impression are taken
Impression traced onto transparencies
Casts of suspects teeth are taken
Comparison between suspect cast and bite mark
Sacco and Vanzetti
April 15th 1920 two men attacked shot and killed two security guards in S Braintree Mass
Escaping with 16k in payroll
Eyewitnesses said the men “looked Italian”
Found many 0.32 bullet shells made by Peters Winchester and Remington
Later Nicola Sacco and Bartolomeo Vanzetti (italian immigrants) were arrested
Sacco and vanzetti 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 the one found at a failed payroll attempt 4 months earlier
Sacco and vanzetti were anarchists and american had had numerous attacks by anarchists with homemade bombs
Arrest of two anarchists showed that at least the police were doing something
Vanzetti first convicted of first payroll attempt (sacco not charged)
Trial them in 1921 both on double murder charges→ their defense sought support from many left-wing groups as an legal attack on their politics
