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Chapter 17 FIREARMS, TOOL MARKS, AND OTHER IMPRESSIONS






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2. Introduction. Structural variations and irregularitiesScratchesNicksBreaksWearRelates bullet to a gun scratch or abrasion mark to a single tooltire track to a particular automobile. FIREARMS AND TOOL MARKS. . Individualization. attainable reality in firearm and tool mark examination. 3.
Chapter 17 FIREARMS, TOOL MARKS, AND OTHER IMPRESSIONS

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1. 1 Chapter 17 FIREARMS, TOOL MARKS, AND OTHER IMPRESSIONS

2. 2 Introduction Structural variations and irregularities Scratches Nicks Breaks Wear Relates bullet to a gun scratch or abrasion mark to a single tool tire track to a particular automobile

3. Individualization attainable reality in firearm and tool mark examination 3

4. 4 Gun Barrel Markings Peculiar to each gun Produced from a solid bar of steel hollowed out by drilling Microscopic drill marks Barrel?s inner surface Randomly irregular Uniqueness to each barrel

5. 5 Gun Barrel Markings Rifling Impressing its inner surface with spiral grooves Lands Surfaces of the original bore remaining between the grooves Grooves guide a fired bullet through the barrel Impart a rapid spin to insure accuracy

6. 6 Gun Barrel Markings Caliber Diameter of the gun barrel Measured between opposite lands Class characteristics of the weapon?s barrel consistent Same number of lands and grooves Same approximate width Same direction of twist

7. 7 Striations Striations Fine lines found in the interior of the barrel Impressed into the metal as the negatives of minute imperfections found on the rifling cutter?s surface Produced by minute chips of steel pushed against the barrel?s inner surface by a moving broach cutter

8. 8 Striations Form the individual characteristics of barrel Inner surface of barrel of gun that leaves its striation markings on bullet passing through it

9. Striations 9

10. Striations 10

11. 11 Bullet Examination No two rifled barrels have identical striation markings Obvious points of comparison Number of lands and grooves Direction of twist Initial stages of examination between an evidence bullet and test-fired bullet Any differences in these class characteristics Eliminate the possibility that both bullets traveled through same barrel

12. 12 The Comparison Microscope Single most important tool to a firearms examiner Two bullets and compared simultaneously within the same field of view Lands and grooves identical widths Longitudinal striations

13. 13 Shotguns Smooth barrel Small lead balls or pellets Gauge: diameter of the shotgun barrel Higher the gauge number: Smaller the barrel?s diameter

14. Striations 14

15. 15 Firing a Weapon Pulling the trigger Release the weapon?s firing pin Strikes primer Ignites the powder Expanding gases generated by the burning gunpowder Propel the bullet forward through the barrel Simultaneously push spent cartridge case or shell back with equal force against the breechblock Shell is impressed with markings Contact with the metal surfaces of the weapon?s firing/ loading mechanisms

16. Handguns 16

17. 17 Cartridge Case Comparison Distinctive signature Firing pin: shape impressed into the relatively soft metal of the primer on the cartridge case Breechblock: rearward thrust Metal to metal contact Ejector mechanism: throws the cartridge or fired case from the firearm Extractor mechanism: cartridge of a fired case is withdrawn from the firing chamber. Magazine or clip: holds the bullets.

18. Gun Samples 18

19. Bullets 19

20. 20 Computerized Imaging Surface characteristics in a manner analogous to automated fingerprint files. National integrated ballistics information network, NIBIN Database files from bullets and cartridge casings Retrieved from crime scenes or test fires from retrieved firearms Links a specific weapon to multiple crimes. Ultimate decision for making a final comparison Will be determined by the forensic examiner Through traditional microscopic methods

21. 21 Gunpowder Residue Unburned and partially burned particles of gunpowder and smoke propelled out of the barrel Muzzle of the weapon close = products deposited onto the target Permits an assessment of the distance from which a handgun or rifle was fired

22. 22 Gunpowder Residue Comparison of the powder-residue pattern Victim?s clothing or skin Test patterns made when the suspect weapon is fired at varying distances from a target Examiner may find enough similarity in shape and density upon which to base an opinion as to the distance from which the shot was fired

23. 23 Gunpowder Residue Weapon in contact /less than 1? from the target Star-shaped (stellate) tear pattern around the bullet hole entrance Surrounded by a rim of a smokeless deposit of vaporous lead Discharge of 12 to 18 inches or less Halo of vaporous lead (smoke) deposited around a bullet hole

24. 24 Gunpowder Residue Distances up to 25? (and occasionally as far as 36?) Scattered specks of unburned and partially burned powder grains without any accompanying soot More than three feet No powder residues, Only visual indication dark ring around the hole, known as a bullet wipe

25. 25 Gunpowder Residue Garments or other evidence relevant to a shooting Surfaces of all items examined microscopically for the presence of gunpowder residue Chemical tests Greiss test Needed to detect gunpowder residues not visible

26. 26 Gunpowder Residue: Shotguns Firing related to test firing Spread of the discharged shot Muzzle to target distances

27. 27 Primer Residue on Hands Gunpowder and primer residues blow back toward the shooter Deposited on the firing hand of the shooter Detection can provide valuable information as to whether or not an individual has recently fired a weapon

28. 28 Primer Residue on Hands Amount of barium and antimony on the relevant portion of the suspect?s hands Thumb web Back of the hand Palm Morphology of particles containing these elements Whether or not a person has fired, handled a weapon, or was near a discharged firearm

29. 29 Serial Numbers Restoration of serial number removed or obliterated by grinding, rifling, or punching

30. Serial Numbers Restoration of serial numbers Chemical etching The metal crystals in the stamped zone under permanent strain Extends a short distance beneath the original numbers 30

31. Serial Numbers Restoration Kit 31

32. 32 Firearm Evidence Collection Hold weapon by the edge of trigger guard or by checkered portions of the grip Precautions taken to prevent accidental discharge of a loaded weapon In most cases unload the weapon When revolver is recovered, the chambers, their positions, and corresponding cartridges must be recorded Marked for identification (usually a tag on the trigger guard) Chain of custody must be established

33. 33 Firearm Evidence Collection Bullets recovered at the crime scene Scribed with the investigator?s initials Either on base or nose of bullet Avoid obliteration of striation markings that may be present on bullet Bullet protected Wrap in tissue paper Place in pillbox or an envelope for shipment Fired casings identified by investigator?s initials near outside or inside mouth of the shell Discharged shotgun shells initialed On the paper or plastic tube remaining on the shell On the metal nearest the mouth of the shell.

34. Tool marks 34

35. Tool Marks any impression, cut, gouge, or abrasion caused by a tool coming into contact with another object 35

36. Tool Marks Class characteristics Size and shape of the tool Minute imperfections on a tool that impart individuality to tool Shape and pattern of imperfections further modified by damage and wear during the life of the tool 36

37. Tool Marks Comparison microscope Compare crime-scene tool marks Test impressions made with suspect tool 37

38. 38 Tool Marks Entire object or the part of the object bearing the tool mark submitted Under no circumstances must crime scene investigator attempt to fit the suspect tool into the tool mark Any contact between the tool and the marked surface may alter the mark and will, at the least, raise serious questions about the integrity of the evidence

39. 39 Other Impressions Shoe, tire or fabric impressions Before any impression moved or otherwise handled Must be photographed (including a scale) Show all the observable details of the impression Readily recoverable item Glass, paper, or floor tile Transported intact to the laboratory Cannot be submitted to the laboratory Preserve the print in a manner similar to lifting a fingerprint

40. Tire Marks 40

41. 41 Other Impressions Shoe and tire marks Soft earth at a crime scene Photography and casting Chemical enhancement can visualize latent or nearly invisible blood impressions. Faint bloody footwear impression Subject track through blood leaving a trail of bloody impressions

42. Footprints 42

43. Footprints 43

44. Footprints 44

45. Footprints 45

46. Bite Marks Bite mark impressions human and animal Skin and foodstuffs Evidence in homicide and rape cases 46

47. Bite Marks 47

48. 48 Points of Comparison Sufficient number of points of comparison or the uniqueness of such points Support finding that both questioned and test impressions originated from one and only one source New computer software and web

49. Summary 49


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