Toolmarks and Firearms

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Chapter 8
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Toolmarks and Firearms

• Chapter Objectives
• Toolmark Definition
• Firearms Examination Background
• Firearms Evidence Examination and Comparison
• Use of Firearms Evidence for Reconstruction
• Serial Number Restoration
• The Firearm and Toolmark Examiner Profession

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I. Toolmark Definition

• A toolmark is a pattern resulting from a harder
  marking device (tool) being forced against a
  softer object
• Toolmarks are usually either striations
  (scratches) or impressions
• Striations result from a sliding action, whereas
  impressions are a result of forcing the tool into
  the surface

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I. Toolmark DefinitionClass Individual
Characteristics

• Toolmarks can have either class or individual
  characteristics and sometimes both
• The basic size and shape of the toolmark is a
  class characteristic and can tell something
  about the kind of tool that made the toolmark
• The individual characteristics are usually
  caused by stria

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I. Toolmark DefinitionResidue from Softer
Object on Tool

• The tool should be first examined using a
  stereomicroscope for residue
• If the residue on the evidence tool is shown to
  be consistent with the surface that was
  scratched, the association of the tool with the
  surface is greatly strengthened
• Material characteristic of the tool may also be
  left in the toolmark

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I. Toolmark DefinitionTypes of Toolmarks

• Three basic types indented, striated, and a
  combination of the two
• Indented marks occur when the tool is pushed into
  the surface
• Striated marks occur when the tool slides across
  the surface
• Combination marks occur when the tool is pushed
  in and then slides across the surface

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I. Toolmark DefinitionCollection of Toolmarks

• A toolmark on a movable object should be
  protected and sent to the lab
• A toolmark on a fixed item, should be thoroughly
  photographed and then cast using silicone rubber
• A suspect tool should never be placed into the
  toolmark, as there is a potential to damage the
  mark and compromise the value of any residue on
  thetool surface

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I. Toolmark DefinitionExamination Comparison
of Toolmarks

• As trace evidence may be present either on the
  tool face or in the mark, it is important to
  collect control samples from an area near the
  mark
• The examiner then attempts to reproduce the
  toolmark by making test impressions in similar
  material
• The toolmark and test impressions are then
  examined using a comparison microscope

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I. Toolmark DefinitionExamination Comparison
of Toolmarks

• The examiner may conclude that
• The toolmark was made by that tool and no other
  tool
• The marks are consistent in class characteristics
  and show a number of other similarities (the tool
  could have made the mark)
• The marks are clearly different (disassociation)
• The marks are not sufficiently clear
  (inconclusive)

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I. Toolmark DefinitionExamination Comparison
of Toolmarks

• To summarize, toolmark examinations may provide
  the following information
• What kind of tool was used to make a mark
• The action which made the mark
• The presence of residue consistent with the
  damaged surface
• The particular tool that was responsible for the
  toolmark

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II . Firearms Examination - Background

• A firearm is a device for accelerating a
  projectile to a high speed, sending it towards a
  selected target
• The energy that provides the driving force for
  the projectile is contained in the cartridge
• A cartridge consists of four basic components
  the case, powder, projectile, and primer

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II . Firearms Examination Background

• Cartridge Components
• the case the container for the other three
  components
• the powder small disks, cylinders, or balls of
  smokeless powder (cellulose nitrate)
• the projectile (bullet) usually made from lead
  or copper-coated lead
• the primer consists of a shock sensitive
  material

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II . Firearms Examination Background

• The firing train process
• The firing pin strikes the primer cup located
  on the base of the case, compressing the cup and
  causing the shock sensitive primer to ignite
• The primer then transfers a spark or flame to the
  powder charge, located in the body of the case
• The burning powder rapidly gives off heat gases
  and the pressure forces the projectile from the
  case and down the barrel of the weapon

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II . Firearms Examination Background

• Rifling
• Rifling refers to the helical (spiral) grooves
  cut or impressed into the barrel of a handgun or
  rifle to cause the exiting projectile to spin
• The raised areas are known as lands and the
  depressed areas, grooves
• A spinning projectile has a flatter trajectory
  and flies a truer path to the target

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II . Firearms Examination Background

• Class Characteristics
• The number of lands and grooves in a firearm
  barrel
• The widths of the lands grooves
• The direction in which the lands and grooves
  spiral (the twist) can be either right or left
• The caliber, the distance between two opposing
  lands in hundreds of an inch (US) or in
  millimeters

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II . Firearms Examination Background

• Computer Databases
• The FBI maintains a central computerized firearm
  file called the General Rifling Characteristic
  file (GRC)
• Firearm examiner can search the file with the
  caliber, number of lands and grooves, their
  widths, and direction of twist for the make and
  model of the firearm that could have fired the
  crime scene bullet

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II . Firearms Examination Background

• Individualizing Features
• Imperfections in the lands during manufacture or
  caused by wear, nicks, scratches etc., will be
  mirrored on the surface of the fired bullets
• These rifling imperfections are unique to each
  barrel and form the basis for projectile-to-barre
  l identification

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II . Firearms Examination BackgroundTypes of
Firearms

• Classification by firing mechanism
• Single shot - fire one round then require manual
  ejection of the cartridge casing before another
  round can be fired
• Semiautomatics - fire one bullet then reload with
  a fresh cartridge each time the trigger is pulled
  and released
• Automatics - continue to fire projectiles until
  the trigger is released

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II . Firearms Examination BackgroundTypes of
Firearms

• Classification by design purpose
• 1. Rifles
• A firearm designed to be fired from the shoulder
  and normally has a long barrel
• Can be single shot, semi-automatic, or fully
  automatic

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II . Firearms Examination BackgroundTypes of
Firearms

• Classification by design purpose
• 2. Handguns
• A firearm with a defined barrel length limit
• Can be single shot, revolvers, or semi-automatic
  pistols
• Revolvers are handguns that have a rotating
  cylindrical cartridge holder, allowing the
  weapon to be fired semi-automatically

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II . Firearms Examination BackgroundTypes of
Firearms

• Semiautomatic Pistols are mechanically analogous
  to the semi-automatic rifles
• Cartridges are placed in a magazine, which is
  located in the grip of the firearm
• Empty cartridge cases are automatically ejected

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II . Firearms Examination BackgroundTypes of
Firearms

• Classification by design purpose
• 3. Shotguns
• Designed to be fired from the shoulder, have
  long barrels, but are generally not rifled
• Shotgun shells, contain either pellets or a slug
  packed into a cartridge along with the powder
  charge and primer

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II . Firearms Examination BackgroundCollection
Preservation of Firearms Evidence

• A firearm must be rendered safe before collecting
  and packaging
• With revolvers, the cylinder position that is
  lined up with the barrel must be recorded
• Bullets and cases recovered from the scene must
  be carefully marked for identification or sealed
  in containers that are marked
• Firearm evidence should always be sent to the lab
  for analysis so that connections can be made to
  other cases using the firearm database

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III. Firearms Evidence Examination
ComparisonExamination for Safety Physical
Condition

• Two key parts of the initial physical
  examination
• ensuring that no cartridges are left in the
  firearm
• sighting down the barrel to verify that it is not
  obstructed
• The weapon is then dry fired (no cartridge) to
  ensure that all the different parts are working
• The basic class characteristics are noted along
  with make, model and serial number

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III. Firearms Evidence Examination
ComparisonFunctionality Obtaining Control
Bullets Cases

• Firearms are test fired to determine
  functionality, proper working of the safety
  features, and to obtain known specimens
• Functionality is the ability of a firearm to fire
  a projectile when the trigger is pulled
• Two or three test firings are done into
  water-filled bullet recovery tanks

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III. Firearms Evidence Examination
ComparisonBullet Cartridge or Shotshell Case
Comparisons

• A comparison microscope is used for the
  examination of bullets and casings
• With bullets, fine striations on the interior of
  the land impressions are examined and compared
• The examiner looks for areas of agreement or
  disagreement between the stria of K specimens
  the evidence

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III. Firearms Evidence Examination
ComparisonBullet Cartridge or Shotshell Case
Comparisons

• With cartridges, the firing pin impressions left
  by the firing pin on the primer cup portion of
  the cartridge can often be individualized to a
  particular firearm
• Cartridge cases can often be associated to a
  particular weapon by breech face markings left
  on primer cup

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III. Firearms Evidence Examination
ComparisonBullet Cartridge or Shotshell Case
Comparisons

• Cases recovered from semiautomatic weapons may
  have extractor /or ejector marks that may
  provide enough information for individualization
• Shotshell cases can acquire firing pin
  impressions, breech face markings, and extractor
  /or ejector marks
• Shotshells can be individualized to a shotgun
  analogous to cartridges from a handgun or rifle

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III. Firearms Evidence Examination
ComparisonFirearm Databases

• NIBIN (National Integrated Ballistic
  Identification Network) contains images of
  bullets and cartridge cases recovered from scenes
  or test fired from seized weapons
• The database can connect different cases through
  bullet and cartridge case evidence

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IV. Use of Firearms Evidence for
ReconstructionRecovered Firearm Fired Evidence
in Reconstructions

• Trigger pull tests are frequently done with
  recovered firearms to evaluate the sensitivity of
  the trigger and evaluate the accidental discharge
  possibility
• Safety mechanisms are evaluated to test if the
  weapon would fire if accidentally dropped
• Bullets may have trace evidence that will provide
  information about the objects encountered during
  its travel

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IV. Use of Firearms Evidence for
ReconstructionMuzzle to Target Distance Powder
Pattern

• Muzzle to target distance estimates are based on
  the comparison of powder patterns produced at
  measured distances using the same firearm and
  ammunition
• Both the distance estimate and the determination
  of the projectile trajectory are important in
  the reconstruction of a shooting incident

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IV. Use of Firearms Evidence for
ReconstructionGSR on Hands Dermal Nitrate,
Lift, Swab, Tape

• Gun shot residue (GSR) refers to materials
  created by the firing of a firearm, particularly
  from the ignition of the primer
• The early dermal nitrate test (paraffin test) has
  been shown to be unreliable and prone to false
  positive results
• Analyzing swabs of the suspects hands for the
  presence of Pb, Ba Sb, is limited by the rapid
  loss of particles, environmental contamination,
  and variable deposits by different firearms

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IV. Use of Firearms Evidence for
ReconstructionGSR on Hands Dermal Nitrate,
Lift, Swab, Tape

• The most reliable method for detecting GSR
  particles requires a Scanning electron microscope
  (SEM) with an energy dispersive X-ray analyzer
  (EDX)
• Samples collected using tape are examined by the
  SEM for particles with characteristic morphology
  (shape)
• Elemental analysis for the presence of Pb, Ba
  Sb is conducted using the EDX

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V. Serial Number Restoration

• The process used to impress the serial number in
  the metal, deforms the crystal structure
  underneath the stamped numbers making it
  susceptible to chemical attack
• The examiner first smoothes-out the damaged area
  then applies an acidic solution to slowly
  dissolve the metal
• The acid preferentially attacks the deformed
  regions revealing the original serial numbers

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VI. The Firearms and Toolmark Examiner Profession

• The primary professional organization of firearms
  and toolmark examiners is the Association of
  Firearms and Toolmark Examiners (ATFE)
• A Scientific Working Group for Firearms and
  Toolmarks (SWGGUN) was recently established to
  examine and recommend standards of practice in
  the firearm and toolmark field