Weapons and Toolmarks

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Weapons and Toolmarks
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Anatomy of a Gun

• Gun barrels are produced by drilling a hole
  through a solid piece of steal
• Drill marks and grooves in barrel will give each
  bullet fired from that gun distinctive striations
• Rifling grooves give the bullet spin
• Caliber is the diameter of the hold in the barrel
  expressed in mm
• Distance between grooves, direction of twist and
  caliber allow identification of type of gun
  bullet was fired from

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Moving Components

• Breechblock
• Supports base of the cartridge in the chamber
• Often finished by hand filing
• characteristic striations on this part contact
  the back of the cartridge leaving individualizing
  markings on the cartridge base
• Firing Pin
• Strikes the cartridge primer to initiate the
  firing process
• Finished on a lathe or filing by hand
• Can transfer striations to the soft metal of the
  primer cap
• Extractor and Ejector
• extracts and ejectors a spent cartridge from the
  chamber
• Metal parts of extractor and ejector can leave
  individualizing markings on cartridge cases
• All of the moving components contact the
  cartridge rather than the bullet can leave useful
  impressions on shotgun shell cartridges

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Manual, Semiautomatic, Automatic

• Single-shot (Manual)
• Bullets must be loaded and casings expelled
• Semiautomatic (Self-loading)
• Use the recoil energy from a fired bullet to
  extract eject expended cartridge casing to load
  a new round into the firing chamber
• The trigger must be activated manually for each
  shot fired
• Two Types (usually revolver type pistols)
• Single-action
• must be manually cocked before firing
• Double-action
• cocked by the pull of the trigger
• Automatic
• Fully automatic weapons will continue to fire
  expend casings as long as the trigger is
  depressed and ammunition available

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Shotguns

• No rifling
• Typically characterized by their gauge
• Gauge is a term which comes from the days when
  all firearms fired spherical lead balls
• Gauge the number of appropriately-sized bullets
  that could be made from one pound of lead
• 10-gauge shotgun 10 lead balls of about the
  same diameter as the barrel could be made from a
  pound of lead
• The higher the gauge number, the smaller the
  diameter of the barrel
• 12-gauge shotgun has a bore diameter of 0.730
  inches
• 16-gauge shotgun has a bore diameter of 0.670
  inches
• Exception.410-gauge shotgun has a barrel 0.410
  inches in diameter

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Bullets

• Lead Alloy
• lead hardened by addition of antimony
• Semi jacketed
• have a copper-alloy or aluminum jacket that
  covers part of the bullet surface
• usually nose is left exposed
• nose configurations encourage penetration
  expansion on impact
• soft-point, hollow-point, bronze point, etc
• Full jacketed
• consist of a lead core surrounded by a jacket of
  copper-nickel alloy or mild steel
• usually core is left exposed at the base of the
  bullet
• Semiautomatic pistols use full jacketed bullets

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Comparison Microscope
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Automated Firearm Search System

• National computerized networks have been
  developed to allow labs to search each others
  data
• DRUGFIRE (FBI)
• concentrates on cartridge markings although
  bullet striae can be stored as well
• IBIS (Integrated Ballistic Identification System)
• developed by the Bureau of Alcohol Tobacco
  Firearms (ATF)
• incorporates two separate software programs
• Bulletproof
• for bullet-specific markings
• Brasscatcher
• for cartridge cases

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Gun Shot Residue

• Swab taken of hands of suspected shooter
• Detect barium and antimony
• Atomic absorption spectrometry
• SEM-EDX
• Morphology of particle determined by SEM
• Scanning electron microscope
• Elemental composition by EDX
• Energy dispersive X-ray analysis

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