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Firearms and Toolmarks. Firearms Summary. Guns (types and manufacturing) Ammunition Firing a gun Collecting Evidence Analyzing Evidence. Firearms. Types of Guns. Firearms. Types of Barrels. Smooth Barrel: Hollow tube.

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firearms summary
Firearms Summary
  • Guns (types and manufacturing)
  • Ammunition
  • Firing a gun
  • Collecting Evidence
  • Analyzing Evidence
slide3

Firearms

Types of Guns

slide4

Firearms

Types of Barrels

Smooth Barrel: Hollow tube

Rifled Barrel: Barrel of gun can be rifled to put spin on bullet; increases accuracy

slide5

Firearms

Lands

Rifled Barrel

  • Inside of barrel has hills (lands) and valleys (grooves)
  • Lands and grooves twist down length of gun

Grooves

slide6

Firearms

Rifled Barrel

  • Inside of barrel has hills (lands) and valleys (grooves)
  • Lands and grooves twist down length of gun
slide7

Firearms

Rifled Barrel (manufacturing)

  • Begin with a hollow tube
  • Lands and grooves made by forcing cutting tools through the tube.
slide8

Firearms

Rifled Barrel (manufacturing)

The minor differences in manufacturing leads to differences in the scratches (striations) on the fired bullet.

slide9

Firearms

Handgun

Designed to be held and fired with one hand

Revolver

Semiautomatic pistol

slide10

Firearms

Revolver

Hammer

Bullets

  • Hammer pulled back manually or when trigger is pulled
  • Hammer hits back of bullet casing; causes explosion
  • Barrel rifled

Trigger

slide11

Firearms

Semiautomatic pistol

Bullets

Hammer

  • Hammer strikes bullet case when trigger pulled
  • Bullet casing automatically ejected.
  • Barrel rifled

Trigger

slide12

Firearms

Rifle

  • Hammer strikes bullet case when trigger pulled

Bullets

Bullets

Hammer

  • Bullet casing manually or automatically ejected.
  • Barrel rifled; fires bullet

Trigger

slide13

Firearms

Projectile

Shotgun

  • Hammer strikes shotshell when trigger pulled
  • Bullet casing manually or automatically ejected.
  • Barrel smooth; fires shot or slug

Trigger

slide14

Firearms > Ammunition

Ammunition

(Bullets)

Bullet

Cartridge

Propellant (inside cartridge)

Primer

slide15

Firearms > Ammunition

Ammunition (Types of Bullets)

Lead or Lead Alloy: softer bullet; scraped up by the lands and grooves of barrel

  • Semijacketed bullet: brass jacket covers sides of lead bullet, except for tip
    • Bullet expands on impact
    • Jacket not scraped up as much
    • Jacket can be shed after hitting target
slide16

Firearms > Ammunition

Ammunition (Types of Bullets)

Full Metal Jacket: lead core completely covered with brass

slide17

Firearms > Ammunition

Shotgun Shell

Shot (pellets)

Cartridge

Wad (plastic)

Propellant (inside cartridge)

Primer

Fires many small round pellets

slide18

Firearms > Ammunition

Primer

  • When hit with hammer, primer creates a small explosion
  • This explosion lights propellant creating a large explosion (propels bullet)
slide19

Firearms > Ammunition

Propellant

  • Modern propellants are smokeless powders composed of nitrocellulose
  • Some contain nitrocellulose and nitroglycerin
slide20

Firearms > Collection

Collection of Firearms evidence

  • Important marks on the outside of the bullet can be damaged during collection.
  • Either collect the item containing embedded bullet or…
  • Use rubber coated forceps to remove bullet
slide22

Firearms > Collection

Collection of Firearms evidence

  • Bullets and cartridges should probably not be physically marked by investigator.

Marks may damage important striations

  • Small evidence should be collected in sealed and marked containers

Pillbox, glass vials

slide23

Firearms > Analysis

Analysis of Firearms evidence

Class Characteristics of bullets:

  • Caliber (size)
  • # of lands/grooves
  • Direction of rifling twist
  • Degree of twist
  • Width of lands/grooves
slide24

Firearms > Analysis

Analysis of Firearms evidence

Class Characteristics of cartridges

  • Caliber (size)
  • Firing pin location
  • Firing pin size/shape
  • Extractor/ejector sizes
  • Relative location of extractor/ejector
slide25

Firearms > Analysis

Analysis of Firearms evidence

If class characteristics of fired bullet and suspected gun match, a test bullet is fired for further analysis.

slide26

Firearms > Analysis

Comparison microscope is used to compare striations on bullets.

slide27

Firearms > Analysis

Comparison of Striations

Striations – tiny scratches on bullet that can be specific to a single gun.

slide28

Firearms > Analysis

Conclusions of analysis

  • If class characteristics and striations match, can say the bullet was likely fired by the suspect gun.
  • If class characteristics (and striations) do not match, the suspect gun can be excluded.
  • If class characteristics match but striations do not, the results are inconclusive.
slide29

Firearms

Range-of-fire estimation

The distance between the gun and victim can be estimated by looking at gunshot residue patterns

Long distance: no GSR on the victim

Close-range: GSR on victim - spread out

Near-contact: GSR on victim - concentrated circle

Contact shot: “Blown out” entrance wound

tool marks
Tool Marks

Tools make specific marks when they cut or scrape a softer surface

Compression tool mark: shows outline of tool (hammer into wood)

Sliding tool mark: parallel striations when tool slides across material (screwdriver or crowbar)

Cutting tool mark: striations when tool cuts through material (scissors)

slide32

Tool Marks

Collection of Tool Marks

  • Notes, sketches, photographs
  • If possible, the tool mark should be taken back to the lab.
  • If not, a cast should be made.

Silicone rubber ideal material

slide33

Firearms > Analysis

Conclusions of analysis

  • If class characteristics and striations match, can say the tool mark was likely made by the suspect.
  • If class characteristics (and striations) do not match, the suspect tool can be excluded.
  • If class characteristics match but striations do not, the results are inconclusive.