Firearms: Class Characteristics

Class Characteristics of Firearms

• Class characteristics serve as blueprint features for firearm identification
• They are controlled by the manufacturer
• They remain consistent across firearms of the same type and model
• They provide the initial basis for identifying a firearm's type or model in ballistic investigations

Significant Firearm Features

• Caliber or Bore Diameter/Gauge refers to the diameter of the barrel and determines the bullet size
• The number of lands and grooves are class characteristics that impart spin for accuracy
• Lands are raised, and grooves are recessed
• Width of lands and grooves are the dimensions of each
• Twist of Rifling/Direction of Twist is the direction of spiral grooves in the barrel, either right or left
• Pitch of Rifling is the rate of twist, measured as inches or centimeters per bullet rotation
• Depth of Grooves refers to the depth within the barrel

Short Firearms

• Steyer type firearms have a code of (4R G=L)
• Smith and Wesson firearms have a code of (5R G=L)
• Colt type firearms have a code of (6L G2X)
• Browning type firearms have a code of (6R G2X)
• Webley type firearms have a code of (7R G3X)

Long Firearms: 2LG - RHT

• Remington rifle cal.30-2LG - RHT

Long Firearms: 4 LG - RHT

• Grease gun cal.45
• Browning automatic rifle cal.30
• Browning machinegun cal.30
• Garand rifle cal.30
• Enfield rifle cal. 30
• Springfield rifle cal.30
• Winchester rifle cal.22
• Winchester rifle cal.30
• Equibman rifle cal.22
• Japanese rifle cal.25
• Mossberg rifle cal.22
• Savage rifle cal.22

Long Firearms: 6LG - RHT

• Martin rifle cal.22
• Remington rifle cal.22
• Thompson submachine gun
• Armalite cal.22

Long Firearms: 8LG - RHT

• Browning machinegun cal.50

Individual Characteristics of Firearms

• Individual characteristics are unique markings that distinguish firearms
• These markings are not intentional design features
• They are random imperfections caused by manufacturing, use, or wear

Causes of Individual Characteristics

• The design of the gun as established by the manufacturer
• The way the firearm is used
• Gun manufacturing processes
• The action of weather

Traces of Individual Characteristics

• Inner surface of the barrel or rifling can be found on the fired bullet
• Breech face of the breechblock can be found on the cartridge case base

Procedures in Making a Barrel

• Drilling uses a long, thin cutting instrument (gun drill) to create holes with high depth-to-diameter ratios, forming the barrel's bore
• Reaming carefully reams the drilled hole to ensure the correct diameter and surface smoothness
• Broaching is a material removal procedure using a broach to remove material progressively

Cut Rifling

• Cut rifling generates rifling in a rifle barrel using a single-bladed, hook-type cutter
• The cutter removes metal to make the grooves, providing tight tolerances
• Metal is removed and rifling grooves are formed one at a time, requiring many passes to attain the correct depth

Button Rifling

• Button rifling produces spiral grooves inside a firearm barrel using a hardened, bullet-shaped tungsten carbide button
• The button has the reverse pattern of the required groove profile
• The button is pushed or pulled through the barrel, displacing the metal and creating the rifling grooves

Hammer-Forging Rifling

• Hammer-forged rifling uses a reusable mandrel with the reverse image throughout the bore
• It creates the appropriate rifling profile
• The plains and grooves are seen in reverse, spiralling the entire length

Ramp Barrel

• The barrel ramp is an important part of a 1911 pistol's barrel assembly
• It ensures that cartridges feed and chamber smoothly and reliably
• It is a section of the barrel machined to match the shape of the bullet as it enters the chamber

Lapping

• Lapping produces a smoother, consistent bore surface, reducing friction between the bullet and barrel rifling during firing

Principles Involved in Firearm Identification

• No two firearms, even of the same make and model, will produce identical markings on bullets and cartridge cases
• Manufacturing and use create unique, microscopic imperfections acting as fingerprints on ammunition

Principle Involved in Bullet Identification

• No two barrels are microscopically alike
• Riflings are engraved on a bullet fired from a rifled gun barrel, which vary in minute details
• Every barrel leaves a "thumb mark or fingerprint" on every bullet that passes through it

Principles Involved in Shell Identification

• The breech face and firing pin of every firearm have individual microscopic individualities
• Every firearm leaves its fingerprint or thumbprint on every cartridge it fires
• Imprints on cartridge cases fired from the same weapon have individual distinctions from the weapon's breech face

Types of Problems in Forensic Ballistic

• Given a fired
• Given a fired cartridge case
• Given a fired bullet and a suspected firearm
• Given a fired cartridge case and a suspected firearm
• Given two or more fired bullets
• Given two or more fired cartridge cases
• Given a firearm

Marks Found on Fired Cartridge Cases

• Chamber marks are found around the cartridge case body caused by irregularities inside the chamber walls
• Shear marks, sometimes called "Secondary pin marks," are found in the primer near the firing pin mark
• Firing pin drag marks are found at the base of the cartridge case near the primer cup or rim cavity
• Extractor marks are on the extracting groove caused by withdrawal from the chamber
• Ejector marks are on cartridge cases fired from automatic firearms near the rim
• Firing pin impressions are indentions created when the firing pin strikes the primer
• Breechface marks are found at the base of the shell
• Pivotal marks are on the shell caused by turning of the cartridge
• Magazine lip marks are on the two sides of the rim caused by the magazine lips during loading

Marks Found on Fired Bullets

• Land marks are on a fired bullet due to contact with the lands of the firearm bore
• Groove marks are caused by the grooves of the barrel
• Skid marks are marks on bullets from a revolver found on the anterior portion
• Stripping marks are on bullets fired from "loose-fit" barrels where the rifling is worn out