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Firearms identification
The science of analyzing firearms, bullets, and cartridge cases to determine if a bullet or cartridge was fired from a particular weapon.
Evidence submitted to Firearms section
Firearms (guns), bullets, cartridge cases, shotgun shells, gunshot residue (GSR) samples, clothing with bullet holes.
Types of firearms (action)
Single-shot, revolver, semi-automatic, fully automatic.
Types of firearms (barrels)
Rifled (handguns, rifles) and smooth (shotguns).
Barrel
The long metal tube of a firearm through which the bullet travels when fired.
Rifling
Spiral grooves cut into the barrel that cause the bullet to spin, increasing accuracy. Rifling leaves unique marks on bullets that can be matched to a specific firearm.
Lands
The raised parts between grooves in a rifled barrel.
Grooves
The cut/low parts in a rifled barrel that determine the bullet's spin.
Caliber
The diameter of a gun barrel measured between opposite lands. It represents the size of the bullet the firearm can fire.
Cartridge
A complete unit of ammunition made up of a bullet, casing, powder, and primer.
Striated action marks
Caused when the bullet moves down the rifled barrel and comes in contact with the lands and grooves, leaving fine scratches.
Impression action marks
Firing pin impression: From the pin striking the primer; Breech marks: From the cartridge slamming against the breech face; Ejector/extractor marks: From mechanical parts that eject or remove the cartridge.
Striations
Microscopic scratches left on bullets as they pass through a rifled barrel. They occur from contact with the lands and grooves.
Ammunition types
Centerfire vs rimfire, handgun vs rifle vs shotgun.
Bullet comparisons
Compare striations on bullets under a comparison microscope to determine if they were fired from the same barrel.
Cartridge comparisons
Compare firing pin, breech, extractor, and ejector marks under a microscope to match to a specific firearm.
Shotguns
Gauge: Bore diameter measurement (lower number = larger bore); Shot: Small lead pellets instead of single bullets.
Class characteristics on cartridges
Caliber, number, width, and direction of rifling (lands and grooves), manufacturer markings, firing pin shape, ejector/extractor design.
AFTE
The Association of Firearm and Toolmark Examiners - the professional organization for firearms examiners.
Examining bullets
Using a comparison microscope to look at striations and match them to a suspect firearm.
Water tanks in firearms examination
To safely recover bullets without damaging markings, allowing for test-firing comparisons.
Equipment for comparing marks
A comparison microscope.
Results from firearms comparisons
Materials bullets are made of
Lead, lead with copper jacket, copper, brass, or steel alloys.
Common bullet shapes
Round nose, hollow point, wadcutter, semi-wadcutter, boat-tail.
Results from cartridge case comparisons
Identification; 2. Elimination; 3. Inconclusive; 4. Unsuitable (too damaged).
Common impression action marks
Firing pin impressions, breech face marks.
Bullet trajectory calculation
Use the formula: Angle of impact = arcsin (minor axis ÷ major axis). Then extend trajectory lines to determine origin.
Weight conversion of bullet
1 gram = 15.43 grains. Multiply grams × 15.43 = grains.