Study Guide: Firearms

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29 Terms

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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.

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Evidence submitted to Firearms section

Firearms (guns), bullets, cartridge cases, shotgun shells, gunshot residue (GSR) samples, clothing with bullet holes.

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Types of firearms (action)

Single-shot, revolver, semi-automatic, fully automatic.

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Types of firearms (barrels)

Rifled (handguns, rifles) and smooth (shotguns).

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Barrel

The long metal tube of a firearm through which the bullet travels when fired.

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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.

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Lands

The raised parts between grooves in a rifled barrel.

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Grooves

The cut/low parts in a rifled barrel that determine the bullet's spin.

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Caliber

The diameter of a gun barrel measured between opposite lands. It represents the size of the bullet the firearm can fire.

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Cartridge

A complete unit of ammunition made up of a bullet, casing, powder, and primer.

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Striated action marks

Caused when the bullet moves down the rifled barrel and comes in contact with the lands and grooves, leaving fine scratches.

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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.

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Striations

Microscopic scratches left on bullets as they pass through a rifled barrel. They occur from contact with the lands and grooves.

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Ammunition types

Centerfire vs rimfire, handgun vs rifle vs shotgun.

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Bullet comparisons

Compare striations on bullets under a comparison microscope to determine if they were fired from the same barrel.

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Cartridge comparisons

Compare firing pin, breech, extractor, and ejector marks under a microscope to match to a specific firearm.

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Shotguns

Gauge: Bore diameter measurement (lower number = larger bore); Shot: Small lead pellets instead of single bullets.

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Class characteristics on cartridges

Caliber, number, width, and direction of rifling (lands and grooves), manufacturer markings, firing pin shape, ejector/extractor design.

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AFTE

The Association of Firearm and Toolmark Examiners - the professional organization for firearms examiners.

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Examining bullets

Using a comparison microscope to look at striations and match them to a suspect firearm.

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Water tanks in firearms examination

To safely recover bullets without damaging markings, allowing for test-firing comparisons.

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Equipment for comparing marks

A comparison microscope.

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Results from firearms comparisons

  1. Identification (match); 2. Elimination (not a match); 3. Inconclusive.
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Materials bullets are made of

Lead, lead with copper jacket, copper, brass, or steel alloys.

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Common bullet shapes

Round nose, hollow point, wadcutter, semi-wadcutter, boat-tail.

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Results from cartridge case comparisons

  1. Identification; 2. Elimination; 3. Inconclusive; 4. Unsuitable (too damaged).

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Common impression action marks

Firing pin impressions, breech face marks.

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Bullet trajectory calculation

Use the formula: Angle of impact = arcsin (minor axis ÷ major axis). Then extend trajectory lines to determine origin.

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Weight conversion of bullet

1 gram = 15.43 grains. Multiply grams × 15.43 = grains.