Chapter 15 Firearms, Toolmarks, Impressions...Firearms, Toolmarks, and Impressions 3 You will be...
Transcript of Chapter 15 Firearms, Toolmarks, Impressions...Firearms, Toolmarks, and Impressions 3 You will be...
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Firearms, Toolmarks,
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Firearms, Toolmarks,
and Impressions
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You will learn:
How class and individual evidence can answer different questions.
How impression analysis is used in
forensic science.
The impact of firearms in the United States.
The importance of microscopic
examination in forensic science.
Objectives
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You will be able to:
Distinguish types of firearms.
Measure individual features of bullets and cartridge cases.
Describe how a handgun works.
Use color tests to find gunshot residue.
Describe the procedure for estimating the distance between muzzle blast and target.
Make casts of different types of impressions.
Objectives, continued
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Activities
Characterization of Bullets and Cartridge Casings
The Greiss Test
The Sodium Rhodizonate Test for Lead Residue
The Corpse in the Closet
Matching Toolmarks
Casting Shoeprints
Relating Shoe Size to Height
Comparing Bite Marks
The Case of the Bitten Bonbon
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Forensic analysis is vital to solve a crime that uses a gun.
In 2004, there were 12,000 homicides in the United States.
Firearms
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Types of Firearms
Handguns (pistols) Revolver
Semiautomatic
Rifles
Shotguns
Air or BB guns
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Ammunition
Cartridge case
Primer
Propellant
Projectile
Components:
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Bullets Made of lead, sometimes jacketed with
brass, copper, or steel
Bullet size—diameter (caliber or gauge)
Shapes
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Rifling
The grooved spirals inside the barrel of a gun that produce lands and grooves on a bullet
Lands and grooves are class
characteristics.
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Striae
Scratches on a fired bullet, like a barcode,
that can serve as individual
evidence, matching bullets or
bullet to a firearm
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Cartridge Case
Usually brass or nickel-clad brass
Head stamps
Rimfire and centerfire cartridges
Class evidence
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Cartridge Case, continued
Individual characteristics
Firing pin marks
Breech marks
Extractor marks
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Features of a Semiautomatic Handgun
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Firearms Evidence
Individual:
Striae
Firing pin marks
Breech marks
Extractor marks
Ejector marks
Chamber marks
Class: Bullet type
Bullet caliber
Bullet weight
Lands and grooves
Rifling
Cartridge case
Head stamp
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Gunshot Residue (GSR)
When a weapon is fired:
Primer and propellant particles blow back toward the shooter.
Combustion products (mostly NO2-), unburned propellant, and
particles of lead follow the bullet, spreading out with distance.
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Distance to Target
The Greiss test converts nitrites to an orange-red color.
Sodium rhodizonate reacts with traces of lead to make purple spots.
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Toolmarks
Class characteristics: type, size, shape
Individual characteristics: features from wear and damage
Tools often used in burglaries may leave a mark.
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Lab Activity: Matching Toolmarks
?
?
Photography and casting are important to match tool with mark.
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Impressions
Class characteristics— manufacturer, type, model, size
Individual characteristics— wear
patterns, nicks, marks, occlusions (like
pebbles or sticks)
Shoeprints
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Impressions, continued
Captured by oblique-angle photography or
chemical enhancement; also by casting in soil,
or lifting.
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Impressions, continued
Treated much the same as shoeprints
Class characteristics involve design, size, type, and model.
Wear and damage cause defects that can lead to individualization.
Tire Treads
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Impressions, continued
TreadMate is a database containing data on more than 5,000 vehicle tires and
tread patterns.
Tire Treads
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Impressions, continued
Result from assault or sexual attack, common in domestic
violence
Individual evidence, if enough impressions
Bite marks were the prime evidence in the conviction of serial killer Ted
Bundy.
Bite Marks
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Impressions, continued
Serial Numbers
Restoration of serial numbers
Items of value may have ID numbers stamped into them.
Grinding is usually used to obliterate identification numbers.
To restore ID numbers on metal, an acid etching solution is employed.
HCl-CuCl2
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Forensics: Debate
Both Sides of the Issue; Gun Control Laws
Should gun sales and ownership be
subject to federal regulation?
Introduction
Pro/con sides
Assertion
Evidence
Personal opinion