Firearms and Bullets

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Firearms and Bullets

• Japhia Jacobo, Kelli Smith, Mylinh Ngo

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Firearms Types

• Firearms can differ in many components
• The way the bullets are loaded and chambered
• The cocking mechanism
• The firing action
• The ejection of used cartridges
• A main distinction is which components are
  automated and which are manual

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Firearms Types Contd

• May be classified by their state of automation
• single-shot (manual)
• semiautomatic
• automatic
• May also be classified by aspects of their
  physical design
• Rifled
• Smoothbore

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Firearms Parts

• Firing and Ejection components of interest
• The breechblock
• The firing pin
• The extractor
• The ejector

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Firearms Firing

• When a trigger is pulled, it acts as a release to
  the weapons firing pin, which causes it to
  strike the primer, which in turn ignites the
  powder
• The expanding gases which are generated by the
  burning gunpowder propel the bullet forward
  through the barrel, while simultaneously pushing
  the spent cartridge case or shell back with equal
  force against the breechblock
• The shell is impressed with markings which are
  made by its contact with the metal surfaces of
  the weapons firing and loading mechanisms.

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Barrel Rifling

• The grooves serve to guide a fired bullet through
  the barrel, imparting a rapid spin to insure
  accuracy
• Has a series of spiral cut-outs that run the
  length of the barrel
• depressions
• grooves
• raised portions
• lands

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Ammunition

• When black powder was used
• Projectiles had relatively low velocities
• Bullets were usually pure lead
• With smokeless powders
• Burn rate and heat production much greater
• Pure lead bullets foul rifling

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Bullet Types

• Soft Lead
• Partial Jacket , Soft Point
• Full Jacket , Pistol and Military Ball
• Hollow Point
• Wad Cutter
• Explosive
• Tracer

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Crime Scene Recovery

• Bullets should only be touched by rubber or
  plastic objects
• Recovered bullets
• scribed with the investigators initials, either
  on the base or the nose of the bullet
• The destruction of striation markings that may be
  present on the bullet must be carefully avoided.
• Bullet must be protected
• wrapping it in tissue paper before placing it in
  a pillbox or an envelope for shipment to the
  crime lab
• Fired casings
• investigators initials placed near the outside
  or inside mouth of the shell
• Discharged shotgun shells
• initialed on the paper or plastic tube remaining
  on the shell or on the metal nearest the mouth of
  the shell

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Lab Analysis

• Serial number restorations
• Examination of Fired Bullets for Same Rifling
  Characteristics
• Examination of Expended Cartridges
• Automated Ballistic Comparisons
• GSR Testing
• Crime Scene Recreation

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Serial Numbers Restoration

• Possible via chemical etching
• the metal is put under permanent strain by the
  stamping process which makes it more chemically
  reactive.
• Two commonly used methods to retrieve serial
  numbers
• Magnaflux method
• Restore serial numbers imprinted on iron or steel
• Iron fillings and light oil
• Acid etching method
• Frys Reagent Used on firearms made of both
  ferrous (iron)
• Vinellas reagent and nonferrous materials (most
  likely aluminum)

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Common Caliber and Rifling
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Bullet Examination

• Bullets can be matched based on class
  characteristics of impressions
• number of lands grooves
• rate of twist
• Direction
• No two guns have the same rifling
• Example
• .32 caliber Smith Wesson revolvers
• 5 lands and grooves twisting to the right
• .32 caliber Colt
• 6 lands and grooves twisting to the left

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Comparison Microscope

• Most important tool to a firearms examiner
• Two bullets can be observed and compared
  simultaneously
• Not only must the lands and grooves of the test
  and evidence bullet have identical widths, but
  the longitudinal striations on each must coincide.

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Bullet Comparison

• One bullet is rotated until a well-defined land
  or groove comes into view
• The other bullet is then rotated in search of a
  matching region
• Analyst must allow for distortion between bullets
  due to mutilation on impact

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Do we have a match?
Yes
No
2
1
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Cartridge Case Impressions

• Highly distinctive signature used for
  individualization for cartridge cases
• The firing pin
• The breechblock
• The ejector
• Extractor mechanisms
• The firing pins shape will be impressed in the
  relatively soft metal of the primer on the
  cartridge case

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The Shotgun difference

• Shotguns have a smooth barrel
• Generally fire small lead balls or pellets that
  are not impressed
• This shot cannot normally be matched back to the
  gun
• The shotshell can be traced to the responsible
  gun via the firing pin

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Automated Firearm Search Systems

• 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)
  ATF
• Bullet Specific Markings
• Cartridge cases
• National Integrated Ballistics Information
  Network (NIBIN)
• bullets and cartridge casings retrieved from
  crime scenes
• test fires from retrieved firearms
• often used to link a specific weapon to multiple
  crimes

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Gunshot and Primer Residue

• Expelled as tiny particles from the barrel of a
  firearm when it is fired
• Contains the heavy metals such as barium (Ba),
  antimony (Sb), lead (Pb)
• Chemical tests such as, Griess test, can be used
  to test gunpowder residues that are not visible

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Gunshot and Primer Residue cont

• Traces of these residues are often deposited on
  the firing hand of the shooter
• Residue traces obtained from the victim can
  determine proximity to shooter
• Examiners measure the amount of barium and
  antimony on the suspects hands
• Determine whether or not a person has handle a
  weapon
• Distance from the shooter to the target

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GSR Detection Technique

• Griess Test
• Neutron activation analysis (NAA)
• Neutron activation only detects antimony and
  barium
• Atomic absorption spectrophotometry (AAS)
• Scanning electrical microscopy with energy
  dispersive analysis (SEM-EDA)

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Griess Test leaded and unleaded

• Griess Test leaded
• Lead and barium detected using sodium rhodizonate
• Less useful with lead-free primers
• Griess Test unleaded
• Rubeanic acid which will react with copper
• More useful with lead-free primers

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Atomic Absorption Spectrophotometry (AAS)

• GSR is collected by swabbing the hands with
  cotton-tipped swabs treated with dilute nitric
  acid
• Swabs then treated with more dilute nitric acid
  and water and injected into the AAS
• It determines the concentrations of lead,
  antimony, and barium released from the primer
  when fired to determine if GSR is present

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Inductively Coupled Plasma (ICP)

• A sample is carried into torch by argon, then
  stripped of metal ions
• The metal ions cool down and produces a photon of
  light with its own wavelength
• Can determine which elements are present by the
  wavelengths given off

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Scanning electrical microscopy with energy
dispersive analysis (SEM-EDA)

• An aluminum stub with an electrically
  conductive adhesive layer is dabbed over the hand
  removing the GSR
• Polyvinyl alcohol is an alternative method to
  collecting the GSR
• A scanning electron microscope is then used to
  reveal a structure, which is then compared to
  known examples of GSR
• This will distinguish large particles of
  partially burned powder and primer residues from
  contaminants

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SEM Spectrum
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Problems when detecting GSR

• All three metals are not necessarily present in
  some rimfire primers
• NAA and FAAS are both quantitative elemental
  analytical methods which do not distinguish the
  source of the metals
• False positives are therefore theoretically
  possible.
• Residues on the palms of the hands may indicate a
  defense gesture, or alternatively the handling of
  a previously fired weapon.
• In suicides residues are often detected on the
  non-firing hand used to steady the muzzle against
  the body.
• Residues of barium alone may be the result of
  contamination with barium rich soil

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Other examinations

• Detection of iron trace on the hands by use a
  ferrozine spray
• Color Change reaction
• Gas chromatography has been used to identify gun
  oils in targets
• Latent fingerprints may be detectable on
  cartridges and expended shell casings

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Crime Scene Reconstruction - Firearms

• Trajectory
• Shooting distance
• Position and location of the victim
• Position and location of the offender
• Sequence of shots
• Direction of shots
• Possibility that the wound(s) could have been
  self-inflicted
• Identification of weapon used may link serial
  cases

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Equipment Laser Trajectory Kit
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Equipment String Trajectory
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References

• Doyle, Scott. An Introduction to Firearm
  Identification, www.firearmsid.com March 2008
• Firearms Tutorial, http//library.med.utah.edu/W
  ebPath/TUTORIAL/GUNS/GUNGSR.html March 2008
• Girard, James E. Criminalistics Forensic Science
  Crime, Jones Bartlett Publishers Inc.,
  Massachusetts, 2008. pp.184-237
• Innes, Brian. Body In Question, Barnes Noble,
  New York, 2005. p.200
• Bell, Suzanne. Forensic Chemistry, Pearson
  Prentice Hall, New Jersey, 2006. p.445
• Saferstein, R. Criminalistics An introduction
  to forensic Science 9th ed.Pearson 2007. Pgs
  460- 480
• Pounder, D. J. Gunshot Wounds lecture notes
  Dept of Forensic Medicine. 1993