Ballistics & Firearms. Ballistics: how bullets behave after being fired from a gun *Study of a projectile in motion. Forensic Firearms Examiners: 1.Analyze bullets & shell casings found at a crime scene 2. Determine what type of weapon fired the bullet
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Ballistics & Firearms
Ballistics: how bullets behave after being fired from a gun
*Study of a projectile in motion
Forensic Firearms Examiners:
1.Analyze bullets & shell casings found at a crime scene
2. Determine what type of weapon fired the bullet
3. Determine trajectory of the bullet
4. Match a bullet or casing to a weapon
5. Match a bullet or casing to a crime scene
***In general, firearms ID determines whether or not a bullet or cartridge was fired from a particular weapon***
How does a gun fire?
Person pulls the trigger.
The firing pin strikes the primer.
The powder ignites.
Gases are released, propelling the bullet forward through the barrel.
The spent cartridge is pushed back against the breechblock.
*Handguns that contain a cylinder that revolves with every pull of the trigger
*After a bullet is fired, the shell casing remains in the cylinder until it is removed
*Loaded using a magazine or clip
*A clip contains cartridges in a spring-loaded device
*Usually loads into the handle of the handgun
*Semi-Automatic weapons eject the empty shell casing from the gun and reloads the next cartridge into the firing chamber
*Utilizes a lever or sliding bolt to eject a spent cartridge and bring the next one into the chamber
*Can also be automatic or semi-automatic
*Fire shells filled with groups of pellets (shot)
*As the shot exits the barrel, they spread out in a circular pattern
*Don’t require a lot of aim or precision
1. Lead bullets
A. Lead is a soft metal. Most lead bullets deform and fragment very easily.
B. Used in low velocity weapons
*Small caliber handguns, some rifles
C. Penetrate less than other bullets, but can still cause damage.
2. Lead Alloy Bullets
A. Alloy: combination of different metals that make them harder.
B. Antimony & Tin are often combined with lead
C. These bullets tend to deform & fragment less, but penetrate deeper.
3. Semi-jacketed bullets
A. Thin layer of brass coating the sides.
B. Soft lead nose, so bullet expands and fragments on impact
C. Can be a hollow point bullet*Nose is slightly hollow
*Deforms & fragments more, causing more damage
D. Used in low velocity & high velocity weapons (.357 magnum, .44 magnum)
4. “Full Metal Jacket”
A. Completely covered in brass, even the nose
B. Used in high velocity guns, such as .44 magnum & high powered rifles
C. Greater penetrating power
D. Can be coated in Teflon, nylon, synthetics to increase speed & penetrability
E. Can penetrate body armor and are known as “cop killers”
Caliber: measurement of the internal diameter of its barrel (not an exact measurement)
*Measured in inches or millimeters
*Example: .38 caliber handgun has a barrel with an approximate internal diameter of 0.38 inches
Gauge: measure of a shotgun’s barrel diameter
*Originally the number of lead balls with the same diameter of the barrel that would make a pound
*Higher the gauge number, the smaller the barrel’s diameter
Shell casing: part of the cartridge that remains after the powder explodes and the bullet is gone
*Contain marks that are important
2. Breechblock Pattern
Breechblock: back wall of the firing chamber
*After a bullet is fired, the casing is forced back against the breechblock, leaving an impression
3. Headstamps: manufacturers stamp on the metal part of the casings
*Can be a manufacturer’s name or logo, caliber or gauge, or cartridge type
4. Extractor & Ejector Marks
*Automatic & Semi-Automatic weapons
*Leave unique scratches & marks on the sides of shell casings when they are “extracted” from the magazine and “ejected” from the weapon
*Gun barrels are “rifled”, meaning they contain spiral grooves that are etched or cut into the inside of the barrels
*This makes the bullet spin as they are ejected, and make them more accurate
Lands & Grooves
Grooves: low points of the rifling
Lands: high points of the rifling, caused by cutting the grooves
Grooves cause the bullet to spin & increase its accuracy
The lands & grooves mark the bullet as it travels along the barrel.
ID Characteristics of Rifling
*FBI maintains a database known as the General Rifling Characteristics file
*Lists land, groove, & twist characteristics of known weapons
*Lands & Grooves are CLASS EVIDENCE!
*Manufacturers of gun barrels cut, stamp, mold, or etch the rifling. They also determine the direction and degree of TWIST.
*Right hand twist, Left hand twist, counterclockwise, clockwise
*Can be used to narrow the possible weapon used
*Each rifled barrel is UNIQUE in its minute characteristics
*Rifling tools cut each barrel a little differently due to wear and damage each time a new barrel is cut or etched
*Repeated firing also wears down the lands & grooves, causing INDIVIDUAL CHARACTERISTICS to the barrel & the bullets fired from it
Older Weapons & Shotguns
*Shotguns & older weapons DO NOT have rifling, so there is no evidence or lands or grooves
*Lands & grooves cut into a bullet as it travels down the barrel, creating unique markings
*These are called striations
*Linear & parallel to the long axis of the bullet
*More noticeable on lead (soft) bullets
*Striation patterns are the same for the same gun
*This is INDIVIDUAL EVIDENCE
*To compare striations, an intact bullet fired from the suspect weapon must be obtained
*Examiner fires the gun into a test-fire chamber (H2O or cotton)
*Test-fired bullet is then viewed with a comparison microscope
Is it a Match?
*Bullets rarely match in every detail, but are still considered important evidence
*Why? Each bullet fired from a gun slightly alters the barrel. It also leaves behind soot and grit. (Rust could be a factor also)
*To be considered a match, you need to find identical patterns on at least 3 consecutive striations on each bullet
TECHNOLOGY TO THE RESCUE!
IBIS : Integrated Bullet Identification System
*Maintained by the ATF
*Used to connect 2 or more cases
2. DRUGFIRE :
*Developed by the FBI
*Shell casings, individual striation patterns