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CRIM B12
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Physical Evidence
Locard’s Exchange Principle states that a criminal always removes something from the crime scene or leaves behind incriminating evidence.
According to Locard, whenever two surfaces come into contact, a transfer of evidence, however slight, will occur.
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big schnoz
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Toolmarks
Tools leave marks by compression, gliding, and cutting.
The microscopic marks left by the tool are known as striations
In theory, a tool can be matched to a toolmark by matching the striation pattern
Over time, the tool surface may change due to usage or corrosion.
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Toolmarks
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Internal ballistics: the bullet’s actions within the gun
Transitional Ballistics: flight of the bullet
Terminal ballistics: the bullet’s affect upon the target
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Firearms
In firearms, microscopic striations are left by components of the weapon.
The semi-automatic handgun, the components are:
• Barrel (revolver)
• Firing Pin (revolver)
• Chamber
• Breech Face (revolver)
• Extractor
• Ejector
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The semi-automatic handgun consists of three main groups, the barrel, frame and slide.
Receiver
Barrel
Slide
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barrel
receiver
slide hammer
grip
magazine well
trigger
triggerguard
magazine release
sightsight lock
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barrel
frame
trigger guard trigger
grip
hammer
cylindersight sight
release
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Rifling and Twist
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Projectile Components
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Bullet Types
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Bullet Deformation
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Striations
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Striations
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Shotgun Shell
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Casing Markings
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Firing Pin Impression
Split Image
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Breech Face Impression
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Extractor Markings