CH 9 - Firearms, Tool Marks, and Other Impressions

Firearms Identification

discipline primarily concerned with determining whether a bullet or cartridge was fired by a particular weapon

Types of Firearms

• Handguns

• Long guns

Handguns

firearms designed to be held and fired with one hand

• pistols

• single shot handguns

• revolvers

Single-Action Handguns

requires the hammer to be manually cocked backward each time before the trigger is pulled in order for the gun to fire

Double-Action Handguns

cock the hammer when the trigger is pulled and then reload the firing chamber around the round is fired

Long guns

designed to be fired while resting on the shoulder

• rifles

• shotguns

The Gun Barrel

the microscopic drill marks left inside the barrel’s inner surface is irregular and unique to each barrel

• Grooves

• Rifling

• Bore

• Lands

• Caliber

Grooves

the cut or low-lying portions between the lands in a rifled bore

Rifling

the spiral grooves formed in the bore of a firearm barrel that impart spin to the projectile when it is fired

Bore

interior of a firearm barrel

Lands

raised portion between the grooves in a rifled bore

Caliber

diameter of the bore, usually expressed in hundredths of an inch or millimeters

• e.g. .22 caliber, 9 mm

Striations

fine lines running the length of the barrel’s lands and grooves

• minute imperfections

  • no two rifled barrels have identical striation markings

Comparing Bullet Markings

must acquire test bullets fired through the suspects barrel

Considerations in Bullet Comparison

• rarely a perfect match

  • final opinion based on judgment, experience, and knowledge of the expert

IBIS

Integrated Ballistic Identification System

• bullet analyzing module

• cartiridge-case-analyzing module

NIBIN

National Integrated Ballistics Information Network

• serves as a screening tool for firearms evidence

  • final comparison made by a forensic examiner

Distance Determination

• process of determining the distance between the firearm and the target

  • usually based on the distribution of powder patterns or spread of shot pattern

Gunpowder and Distance Determination

• needed for pleas of self-defense or suspected suicide cases

Powder Residues on Garments

• microscopic examination

  • gunpowder residue may be identifiable by its color, sizes, shapes

• Greiss Test: chemical test used to examine patterns of gunpowder residues around bullet holes

Swabbing for Primer Residues

• front and back of hand swabbed separately

• low rate of positive findings

SEM Testing for Primer Residues

• adhesive containing stubs applied to surface repeatedly to get samples from suspect’s hands

• Scanning Electron Microscope

• better than swabbing, but more time consuming

Collection and Preservation of Firearms

• hold the weapon by the edge of the trigger guard or checkered portion of the grip

• Safety

  • in most cases, unload the weapon

    • note the weapon’s hammer and safety position prior

    • note location of all fired and unfired ammunition

Collection and Preservation of Ammunition

• caution when removing a lodged bullet

  • break away surrounding support material

  • avoid direct contact with projectile

Collection and Preservation of Gunpowder Deposits

• clothing of a firearms victim must be preserved

  • least disturbance around area of bullet hole

Comparing Tool Marks

• Class Characteristics

  • size

  • shape

• Typically, no individual characteristics

  • expect when there are nicks and breaks in the tool

Collecting Tool Mark Evidence

• when practical, the entire object or part of the object bearing a tool mark should be submitted to the lab

  • otherwise: photograph to scale and make a cast

• remember there is a possibility of trace evidence