Chapter 9 Firearms, Tool Marks and Other Impressions

Firearms

Weapons designed to discharge projectiles.

Rifling

Spiraled grooves inside a barrel for bullet stabilization.

Class characteristics

Shared features among bullets or cartridge cases.

Individual characteristics

Unique markings from specific firearms or tools.

FBI search systems

Databases for firearms-related investigations.

ATF search systems

Bureau's database for tracking firearms.

Muzzle-to-target distance

Distance from firearm to shooting target.

Firearms evidence collection

Procedures for preserving and analyzing weapon evidence.

Serial number restoration

Techniques to recover obliterated firearm serial numbers.

Tool mark identification

Linking tools to marks left on surfaces.

Footwear impressions

Marks left by shoes at crime scenes.

Tire impressions

Marks made by vehicle tires on surfaces.

Single-shot handgun

Fires one round, requires manual loading.

Revolver

Handgun with rotating cylinder for multiple rounds.

Semiautomatic pistol

Automatically reloads after each shot fired.

Single-action firearm

Requires manual cocking before firing.

Double-action firearm

***** and reloads when trigger is pulled.

Swing-out revolver

Cylinder swings out for easy loading.

Break-top revolver

Barrel swings down for loading access.

Solid-frame revolver

Cylinder rotates within a fixed frame.

Firing chamber

Holds cartridge aligned with the barrel.

Cartridge case

Container for bullet, gunpowder, and primer.

Gunpowder residues

Chemical traces from discharged firearms.

Crime scene impressions

Marks left by objects at a crime scene.

Casting impressions

Creating a mold of a mark for analysis.

Lifting impressions

Transferring marks from surfaces for examination.

Comparison of impressions

Analyzing similarities between different marks.

Hammer

Part of a firearm that strikes the firing pin.

Cylinder

Rotating part of a revolver holding cartridges.

Cartridge

A casing containing a projectile and propellant.

Solid-Frame Revolver

Revolver with a fixed frame for durability.

Loading Gate

Opening for inserting cartridges in a revolver.

Handguns

Firearms designed to be held and fired with one hand.

Semiautomatic Pistol

Handgun that fires one round per trigger pull.

Removable Magazine

Container for cartridges, often in the grip.

Firing Component

Mechanism that strikes the cartridge to fire.

Slide

Top part of a semiautomatic pistol that loads rounds.

Gases

Byproducts of firing used to cycle the action.

Long Guns

Firearms designed to be fired from the shoulder.

Rifles

Long guns with a rifled barrel for accuracy.

Shotguns

Smooth bore firearms designed to fire multiple pellets.

Minimum Barrel Length (Rifle)

U.S. law requires 16 inches for rifles.

Single-shot Rifle

Rifle requiring manual loading for each shot.

Lever-action Rifle

Rifle using a lever to cycle cartridges.

Bolt-action Rifle

Rifle using a bolt to load and eject cartridges.

Pump-action Rifle

Rifle using a slide to cycle cartridges.

Auto-loading Rifle

Rifle that automatically cycles using gas or recoil.

Minimum Barrel Length (Shotgun)

U.S. law requires 18 inches for shotguns.

Trigger Pull

Action of pulling the trigger to fire a shot.

Firing Chamber

Part of the firearm where the cartridge is fired.

Cock the Gun

Prepare the firing mechanism for firing.

Extract and Eject

Remove spent cartridge from the chamber.

Load Fresh Cartridge

Insert a new cartridge into the firing chamber.

Gas Pressure

Force used in auto-loading firearms to cycle action.

Recoil

Backward movement of a firearm when fired.

Single-shot shotgun

A shotgun that holds one cartridge only.

Over-and-under shotgun

Shotgun with two barrels stacked vertically.

Double-barrel shotgun

Shotgun with two barrels side by side.

Bolt-action shotgun

Shotgun operated by manually cycling the bolt.

Lever-action shotgun

Shotgun operated by a lever mechanism.

Pump-action shotgun

Shotgun requiring manual pumping to chamber rounds.

Auto-loading shotgun

Shotgun that automatically chambers the next round.

Cartridge case

Holds all components of a small-arms cartridge.

Primer

Ignites the propellant in a cartridge.

Propellant

Gunpowder that creates gas pressure when ignited.

Bullet

Projectile fired from a firearm.

Brass composition

70% copper and 30% zinc for cartridge cases.

Cartridge case function

Seals chamber against gas escape when fired.

Gas pressure expansion

Causes cartridge case to fit chamber tightly.

Tempered brass

Allows cartridge case to spring back after firing.

Straight cartridge case

Commonly used in handgun ammunition.

Bottleneck cartridge case

Allows more powder in a compact design.

Tapered cartridge case

Gradually narrows from base to mouth.

Rimmed cartridge case

Has a rim larger than the base diameter.

Semi-rimmed cartridge case

Rim projects slightly beyond the base.

Rimless cartridge case

Rim diameter equals cartridge case diameter.

Rebated cartridge case

Rim smaller than the base, for extraction only.

Belted cartridge case

Features a raised belt for extraction support.

Head stamp

Identifies cartridge manufacturer and caliber.

Centerfire cartridge

Primer located in the center of the base.

Rimfire cartridge

Primer located in the annular rim area.

Rimfire

Priming mixture located in the cartridge rim.

Centerfire

Primers located in the cartridge center.

Primers

Ignite the powder charge in cartridges.

Boxer Primer

Single flash hole, includes brass anvil.

Berdan Primer

Two flash holes, no integral anvil.

Lead Styphnate

Common component in U.S. priming mixtures.

Barium Nitrate

Oxidizer in primer compositions.

Antimony

Element tested in GSR analysis.

Rimfire Cartridges

No primer assembly; composition in rim.

Black Powder

Mixture of charcoal, sulfur, potassium nitrate.

Charcoal

Fuel component of black powder.

Potassium Nitrate

Oxygen supplier in black powder.

Sulfur

Enhances density and ignitability of black powder.

Gas Production

44% of black powder's weight upon burning.

Solid Residues

56% of black powder's weight; creates smoke.

Booster Charge

Black powder used in large-caliber applications.

Velocity Limit

Cartridge velocities seldom exceed 1400 ft/sec.

Volumetric Inefficiency

Black powder requires large volume for energy.

Corrosive Residue

Black powder by-products accelerate firearm wear.