FS Physical Methods Exam #3

Firearms Identification

Determines whether a bullet or cartridge was fired by a particular weapon and involves bullet comparison, restoration of serial numbers, and the ID of gunpowder and primer residues

Ballistics

The study of a projectile in motion

Types of Firearms: Handguns

• revolvers

• Semiautomatic pistols

• Submachine guns

Types of Firearms: Shoulder Fire Guns

• Shotguns

• Rifles

Rifled Bore

lands and grooves on the bore; imparts a spin upon the projectile, resulting in improved accuracy

Lands

• Area between the grooves

• Raised areas on the bullets caused by indentations of the bore

Grooves

• Scratch patterns on the bullet given by the rifling of the bore that is used to identify the weapons manufacturer

• indentations on the bullet caused by raised areas on the bore

Land and Groove Characteristics

• class characteristics

• The number of and the direction of the twist are gun manufacturer characteristics

Caliber

• the internal diameter of the barrel

• As it increases the kinetic energy of the bullet fired from the gun also increases

• Larger diameter = larger bullet

Gauge

Refers to the diameter of a barrel of a shotgun

Individual characteristics for Rifling Markings

• Striation Markings

• Minute imperfections in rifling

• Cannot be duplicated in any two barrels

Ammunition

The complete loaded assembly

Cartridge Components (rifles and handguns)

• primer

• Gunpowder

• Bullet

Shotgun Shells

• bore sized projectiles (slugs), pellets of large shot, or many tiny pellets (BBs)

• Gunpowder

• Primer

Internal Ballistics

• refers to events that transpire within the firearm

• Ignition of primer and propellant, acceleration of projectile, pathway through the barrel and exit

External Ballistics

• the events that occur after the bullet leaves the barrel of the gun before it strikes its target

• Gravity, aerodynamic drag, wind force

Terminal Ballistics

The events that occur when the bullet strikes the target

How a Firearm Works

1. Firing pin hits the base of the cartridge igniting the primer powder

2. Primer powder sparks through the flash hole to the main propellant supply

3. Pressure of explosion pushes the bullet from the casing into the barrel

4. Bullet follows the lands and grooves spiraling out of the barrel

Serial Number Restoration: Magnaflux Method

• works on iron or steel and is nondestructive

• Grind the surface to smooth it out

• A mixture of fine iron filings and light oil is applied while placing a magnet on the opposite side

• Filings arrange themselves in the oil to produce a shadow of the serial number

Serial Number Restoration: Acid Etching Method

• used on ferrous (iron) and nonferrous materials

• Grind the surface to smooth

• Apply Fry’s Reagent (ferrous) or Vinella’s Reagent (nonferrous) to the area

• Dissolves away extraneous scratches and markings, revealing the serial number below (destructive)

Collection and Preservation of Firearm Evidence

• ensure firearm is rendered safe (safety over evidence)

• If the firearm is loaded, the number and type of rounds are noted

• Firearms should be fixed with zipties in appropriately sized boxes

Collection and Preservation of Ammunition

• each bullet/cartridge should be packaged separately in a paper envelope

• If removing bullets from walls/buildings, use rubber forceps to touch the bullet to prevent other markings

• Occasionally may be best to remove the wall and bring to lab

Lab examination of Firearm Evidence

Use of a comparison microscope in attempt to match striations of a known bullet and a questioned bullet

What can be determined from Firearms Evidence?

• if bullets came from a particular known weapon

• If bullets or casing came from the same firearm

• How many guns were involved

• Types of gun fired based on class characteristics

Integrated Ballistics Identification System (IBIS)

• the “tool”

• Contains specialized hardware and software imaging technology

• Captures 2D bullet images for the NIBIN database

• Helps to compare spent cartridges and shotshells

National Integrated Ballistics Information Network (NIBIN)

• the “system”

• Managed by the ATF

• Database of cartridge cases

Centerfire

• More reliable and useful for large game

• In pistols, rifles, and shotguns

Rimfire

• less reliable but cheaper and readily available

• Useful for smaller game

• Used primarily in long rifles

Composition of GSR depends on:

• formulation of primer

• Formulation of propellant/gunpowder

• Barrel scrapings

• Composition of a projectile

GSR primarily composed of

• barium

• Antimony

• Lead

Ammunition Primers: Initiator

• primary explosive

• Usually lead azide and mercury fulmite

Ammunition Primer: Oxidizer

• provides O2 for combustion

• Usually barium nitrate or potassium nitrate

Ammunition Primers: Fuel

• flame producer

• Usually antimony sulfide or lead thiocyanate

Ammunition Primers: Sensitizing

• creates friction to transfer sparks to gunpowder

• Commonly used is tetracene, pentaerythritol, or TNT

Gunpowder: Black Powder

• potassium nitrate, sulfur, and charcoal

• Converts solid products to gaseous CO2 and N2

• Solid formation causes buildup in the firearm

Gunpowder: Smokeless Powder

• Cordite and Ballistite (nitrocellulose and nitroglycerine)

• Combusts to produce mostly CO2 and N2

• No solid formation

Components of the Core of the Bullet

• lead and antimony

• Or lead and tin

• Provides weight, density, and KE transfer

• 
  

Components of the Jacket of the Bullet

• copper alloys with zinc or tin

• Steel

Detectable Inorganic components of GSR

• Lead

• Barium

• Antimony

• Copper

• Iron

Detectable Organic Components of GSR

• Nitrocellulose (semi burned and unburned)

• Nitroglycerine

Location of GSR on a Suspect

• Hands (#1 spot for evidence)

• Clothing

• Arms

• face

• Hair

As distance from the shooter increases

The presence of GSR decreases

GSR Evidence (Transient)

• extremely fragile and should be collected within three hours

• Use of hand protection bags to preserve valuable evidence and prevent the loss of GSR

GSR Evidence - Clothing

• wrap carefully in clean paper and folded as little as possible to prevent dislodging powder products

• Package each item separately

Collection of GSR

• Swabbing (most common) with a special kit of nitric acid

• Aluminum stubs that contains electrically conductive adhesive

Color Tests: Modified Greiss Test

• presumptive for gunpowder

• Tests for nitrates generated during combustion of gunpowder

• Positive test results in a rosy-orange reddish color

• Use of treated photo paper on the sample steamed with diluted acetic acid solution

• conducted first in protocol so that it does not interfere with lead test results

Color Tests: Rhodizonic Acid Test

• tests for lead and other heavy metals (primers made of lead styphnate)

• Positive test results lead to a scarlet red color change

• Changes to violet color when adding HCl to differentiate lead from other metals

Confirmatory Tests for GSR: Atomic Absorption Spectroscopy (AAS)

• contains a lamp dependent on metal being detected

• Monochromator that selects a specific wavelength for specific metal

• Detector which turns wavelength into an electrical signal to send to a computer to be read

Confirmatory Tests for GSR: Scanning Electron Microscopy

• Detects backscatter electrons through x-rays

• Produces an image

• Detects what particles are present

Rubeanic Acid Test

• detects copper and other metals like nickel

• changes to dark green with copper

• changes to blue-pink with nickel