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

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Presentation on theme: "Firearms and Toolmarks. Firearms Summary Guns (types and manufacturing) Ammunition Firing a gun Collecting Evidence Analyzing Evidence."— Presentation transcript:

1 Firearms and Toolmarks

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

3 Types of Guns Firearms

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

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

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

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

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

9 Handgun Designed to be held and fired with one hand RevolverSemiautomatic pistol Firearms

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

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

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

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

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

15 Ammunition (Types of Bullets) 1. Lead or Lead Alloy: softer bullet; scraped up by the lands and grooves of barrel 2. 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 Firearms > Ammunition

16 Ammunition (Types of Bullets) 3. Full Metal Jacket: lead core completely covered with brass Firearms > Ammunition

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

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

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

20 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 Firearms > Collection

21 Fired Bullets Firearms > Collection

22 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 Firearms > Collection

23 Analysis of Firearms evidence Class Characteristics of bullets: Caliber (size) # of lands/grooves Direction of rifling twist Degree of twist Width of lands/grooves Firearms > Analysis

24 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 Firearms > Analysis

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

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

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

28 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. Firearms > Analysis

29 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 Firearms

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 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 Tool Marks

33 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. Firearms > Analysis


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