1 CRIM B12. 2 Physical Evidence Locard’s Exchange Principle states that a criminal always removes something from the crime scene or leaves behind incriminating.

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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