Chapter 9 Forensics

Firearm identification

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

Two categories of firearms

• handguns

• long guns

Handguns

meant to be held and fired with one hand

• single shot pistol- fire only one round at a time

• revolver- feature several firing chambers within a revolving cylinder

  • swing out revolvers, break top revolvers, solid frame revolvers

• semi automatic pistol- feature a removeable magazine

  • fire one shot per trigger pull

Long guns

meant to be fired while resting on shoulder

• rifles- feature a barrel with lands and grooves

• shotguns- ammunition contains numerous ball shaped projectiles called slug

Cartridge

a single unit of ammunition consisting of the case, primer, and propellant with one or more projectiles

• also applies to a shot shell

Cartridge Case

the container for all the other components which comprise a cartridge

Bullet

the projectiles which is expelled from the barrel

Caliber

diameter of the bullet, specifically the projectile, commonly called the slug

• measured between opposite lands

  • mostly recorded in hundreths of an inch

Jacket

the metallic cover that surrounds the core of the bullet

Core

the inner portion of a jacketed bullet, usually lead

Gun barrel

solid bar of steel that has been hollowed out

• inner surface shaped by a process known as rifling

Lands

surfaces remaining of the original bore (raised potions)

Grooves

guide a bullet down the barrel (cut/low lyng portions- impart spin therefore increase accuracy)

Rifling

accimplished by cutting all grooves in one pass with a cutter known as a broach,

or pressing all the grooves at once onto the barrel with a tool (button) or

hummer forging the barrel using a mandrel containing the reverse image of the rifling

Gun Barrel Markings

• common between firearms manufactured in same process

  • # of lands and grooves

  • left or right twist

  • caliber

• differences

  • striations- fine lines found in the interior of the barrel

Bullet comparisons

test fires created for comparison

1. class characteristics

2. individualziatio

Class characteristics in bullet comparisons

• # lands/grooves

• twist

• caliber

• ** Any differences in these class characteristics immediately serve to eliminate the possibility that both bullets traveled through the same barrel

Individualization in bullet comparisons

• Done using a comparison scope

  • striations

    • rarely a perfect match bc striation markings on barrels are not permanent, subject to continuing change

  • no minimum number of points to make a match

Shotgun

generally have a smooth barrel (no rifling) so projectiles that pass through the barrel are not impressed with any markings

• fire small lead balls or pellets

Guage

size designation of a shotgun

• high gauge, smaller diameter

• guage is the number of lead balls with the same diamater as the barrel that make one pound

Firing a weapon

• trigger pulled releases firing pin

• firing pin strikes primer which ignites powder

• gases propel bullet forward and cartrdige case back against the breech face

Cartridge Case Comparisons

compare markings left by the firing pin, breechface, ejector, and extractor

• markings left on cartridge cases may give possible type or make of the firearm in the absence of suspect weapon

• markings can be individualizing

• shape of firing pin will be impressed into the relatively soft metal of the primer on the cartridge

Breechface

rear part of the firearm barrel

Extractor

mechanism by which a cartridge is withdrawn from the chamber after firing

Ejector

mechanism that throws the fired cartridge case from the firearm

Drugfire

–developed for the FBI and emphasized examination of unique markings on cartridge cases, but could also store images of highly characteristic bullet striations

IBIS

developed for the Bureau of Alcohol, Tobacco, Firearms, and Explosives processed digital microscopic images of identifying features found on bullets and cartridge case

NIBN (National Integrated Ballistics Information Network)

•produces database files from bullets and cartridge casings retrieved from crime scenes or test fires from retrieved firearms, often linking a specific weapon to multiple crimes.

• more than 200 law enforcement agencies use NIBN

• NIBN is only a screening tool

Distance determination

process of determining the distance between the firearm and target (need suspect firearm)

• -If the muzzle is close, the residue is deposited on the target

  -The distribution of gunpowder particles and other discharge residues around a bullet hole permits an assessment of the distance from which a handgun or rifle was fired.

Distance determination w/o suspect firearm

contact or less than 1 inch- star shaped tear pattern around bullet entrance surrounded by rim of smoleless deposit

12-18 inches- halobof vaporous lead deposited around bullet hole

up to 25 in- presence of scattered specks of unburned and partially burned pwder grains without any accompanying soot

3 ft- no deposit or residue, onl visible indication is dark ring around the hole (bullet wipe)

Distance determination with shotgun

• Measure the spread of the discharged shot

  • close range (up to 5 ft)- shot charge enters the target as a concentrated mass producing a hole a little larger than the bore of the barrel

  • 12 guage gun shotgun, pattern distribution increases by 1 inch for every yard of distance

Powder residue on clothing

• microscopic examination for GSR

• infrared photography

• chemical tests (for those residues that are not visible)

Chemical tests

greiss test

sodium rhoizonate

Greiss test

develops patterns of gunpowder residues (nitrates) around bullet holes

Nitrates

chemical product that results from the incomplete combustion of smokeless powder

• can compare pattern on evidence to test fires to approximate distance as well

Sodium Rhodizonate

spray to detect any lead residue

Old way to find primer residue

• dermal nitrate test-hot paraffin was applied to suspects hands, removed, and tested

  • lack of specificity

Current way of finding primer residue

• Examiners measure the amount of barium and antimony on the relevant portion of the suspect’s hands

  • collection done using adhesive stubs or swabbing

Serial Number restoration

serial numbers are usually stamped onto metal and hard steel dies

• Restoring serial numbers is possible through chemical etching because the metal crystals in the stamped zone are placed under a permanent strain that extends a short distance beneath the original numbers.

  
  

Process for serial number restoration

• clean of dirt and oil

• polish

• apply reagent (hydrochloric acid, copper chloride, water for steel

Firearm evidence collection

• safety first priority

• prevent loss of evidence

• unload

• record all necessary info

• mark firearm evidence to establish chain of custody

• properly package

Ammunition collection

• bullets, cartridges, cases, and shotgun shells placed in marked container

• protect any striations/markings on evidence (wrap in bubble wrap before placing in box)

• remove supporting material if collecting bullet from a surface

Tool mark

any impression, cut, guage, or abrasion caused by a tool coming into contact with another object

• class characteristics- size/shape of tool

• individual characteristics- minute imperfections of the tool

Tool mark collection

when practical, the entire object bearing tool mark should be submitted to crime lab

if impractical, take pictures and make cast of the mark using liquid silicone

*DO NOT fit the tool into the tool mark

Impressions

• must be photographed before moved

• if impression is on recoverable item, trasnport evidence to lab

• may be lifted similar to fingerprints

• electrostatic lifting device (dust impressions)

Impression casting

shoe and tire marks impressed into soft earth

• photo first

• use dental stone to make cast (dry 24-48 hrs)

• snow impression wax- sprayed on impressions in snow

Impression enhancing

•In areas where a bloody footwear impression is very faint or where the subject has tracked through blood leaving a trail of bloody impressions, chemical enhancement can visualize latent or nearly invisible blood impressions.

Bitemarks

bite mark impressions on skin and food have proven to be important evidence in a number of homicide and rape cases.

–need sufficient number of points of similarity between test and suspect bite marks

Impression comparison

•evidential value determined by presence of class and individual characteristics

SICAR

shoeprint image capture and retrieval. NOT maintained by gov agency as others.

What are the 3 class characteristics for a gun barrel?

• caliber

• number of lands/grooves

• direction/twist of lands and grooves

Why might striations on a bullet fired from the same gun vary slightly?

• The presence of grit and rust can alter the markings on bullets fired through the same barrel

• Striation markings on a barrel are not permanent structures; they are subject to continuing change and alteration through wear as succeeding bullets traverse the length of the barrel.

Besides the barrel, what parts of a firearm may leave distinctive markings on a shell cartridge?

• firing pin, breechface, extractor, ejector, magazine, the clip, fire chamber walls can all leave distinctive markings

Why does a firearms examiner test-fire bullets from a suspect barrel?

because there is no other practical way to directly compare the markings on the fired bullet and those found within a barrel.