firearms, toolmarks, and impressions 1. 2 lesson 1 – firearms lesson essential questions: how are...
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Firearms, Toolmarks, and Impressions
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Lesson 1 – Firearms
Lesson Essential Questions:
• How are firearms and bullets distinguished from one another?
Vocabulary: Caliber, bore, gauge, shot, slug, grains, rifled, lands, grooves, striae, cannelures, head stamp, breech, extractor, ejector, magazine, chamber
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Forensic analysis is vital to solve a crime that uses a gun.
The vast majority of
U.S. homicides involve
guns. And they are more
powerful than ever.
Lansing State Journal, July 2007
In 2004, there were 12,000 homicides in the United States.Forensic Firearm Examiner
Firearms
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Types of Firearms
Handguns (pistols) Revolver Semiautomatic
Rifles
Shotguns
Air or BB guns
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Handgun
Designed to be held and fired with one hand
Revolver Semiautomatic pistol
Firearms
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Revolver
Hammer pulled back manually or when trigger is pulled
Hammer hits back of bullet casing; causes explosion
Hammer Bullets
Trigger
Barrel rifled
Firearms
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Semiautomatic pistol
Bullet casing automatically ejected.
Hammer Bullets
Trigger
Hammer strikes bullet case when trigger pulled
Barrel rifled
Firearms
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Rifle
Hammer strikes bullet case when trigger pulled
Bullet casing manually or automatically ejected.
Bullets
Trigger
Bullets
Hammer
Barrel rifled; fires bullet
Firearms
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Shotgun
Hammer strikes shotshell when trigger pulled
Bullet casing manually or automatically ejected.
Projectile
Trigger
Barrel smooth; fires shot or slug
Firearms
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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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Smooth Barrel: Hollow tube
Firearms
Types of Barrels
Rifled Barrel: Barrel of gun can be rifled to put spin on bullet; increases accuracy
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Rifled Barrel
Inside of barrel has hills (lands) and valleys (grooves)
Lands
Grooves
Lands and grooves twist down length of gun
Firearms
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Rifled Barrel (manufacturing)
Begin with a hollow tube Lands and grooves made by forcing cutting
tools through the tube.
Firearms
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Rifled Barrel (manufacturing)
The minor differences in manufacturing leads to differences in the scratches (striations) on the fired bullet.
Firearms
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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 typeBullet caliberBullet weightLands and groovesRiflingCartridge caseHead 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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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
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Lesson 2 - Toolmarks
Lesson Essential Questions:
• How are toolmarks distinguished from one another?
Vocabulary: toolmarks
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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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Lesson 3 - Impressions
Lesson Essential Questions:
• How are impressions used in forensic science?
Vocabulary: outsoles, gait, wheelbase, stance
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3 Main Types of Impressions1. Footwear marks or shoeprints
2. Tire Treads
3. Bite marks
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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
Shoeprints
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