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Firearms and Toolmarks

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 and Toolmarks

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  1. Firearms and Toolmarks

  2. Firearms Summary • Guns (types and manufacturing) • Ammunition • Firing a gun • Collecting Evidence • Analyzing Evidence

  3. Firearms Types of Guns

  4. Firearms Types of Barrels Smooth Barrel: Hollow tube Rifled Barrel: Barrel of gun can be rifled to put spin on bullet; increases accuracy

  5. Firearms Lands Rifled Barrel • Inside of barrel has hills (lands) and valleys (grooves) • Lands and grooves twist down length of gun Grooves

  6. Firearms Rifled Barrel • Inside of barrel has hills (lands) and valleys (grooves) • Lands and grooves twist down length of gun

  7. Firearms Rifled Barrel (manufacturing) • Begin with a hollow tube • Lands and grooves made by forcing cutting tools through the tube.

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

  9. Firearms Handgun Designed to be held and fired with one hand Revolver Semiautomatic pistol

  10. Firearms Revolver Hammer Bullets • Hammer pulled back manually or when trigger is pulled • Hammer hits back of bullet casing; causes explosion • Barrel rifled Trigger

  11. Firearms Semiautomatic pistol Bullets Hammer • Hammer strikes bullet case when trigger pulled • Bullet casing automatically ejected. • Barrel rifled Trigger

  12. Firearms Rifle • Hammer strikes bullet case when trigger pulled Bullets Bullets Hammer • Bullet casing manually or automatically ejected. • Barrel rifled; fires bullet Trigger

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

  14. Firearms > Ammunition Ammunition (Bullets) Bullet Cartridge Propellant (inside cartridge) Primer

  15. 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

  16. Firearms > Ammunition Ammunition (Types of Bullets) Full Metal Jacket: lead core completely covered with brass

  17. Firearms > Ammunition Shotgun Shell Shot (pellets) Cartridge Wad (plastic) Propellant (inside cartridge) Primer Fires many small round pellets

  18. Firearms > Ammunition Primer • When hit with hammer, primer creates a small explosion • This explosion lights propellant creating a large explosion (propels bullet)

  19. Firearms > Ammunition Propellant • Modern propellants are smokeless powders composed of nitrocellulose • Some contain nitrocellulose and nitroglycerin

  20. 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

  21. Firearms > Collection Fired Bullets

  22. 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

  23. 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

  24. 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

  25. Firearms > Analysis Analysis of Firearms evidence If class characteristics of fired bullet and suspected gun match, a test bullet is fired for further analysis.

  26. Firearms > Analysis Comparison microscope is used to compare striations on bullets.

  27. Firearms > Analysis Comparison of Striations Striations – tiny scratches on bullet that can be specific to a single gun.

  28. 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.

  29. 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

  30. 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)

  31. Tool Marks

  32. 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

  33. 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.

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