Firearms, Tool Marks, and Other Impressions

1
Firearms, Tool Marks, and Other Impressions

• Chapter 15

2
Chapter Objectives

• Describe the techniques for rifling a barrel
• Recognize the class and individual
  characteristics of bullets and cartridge cases
• Understand the use of the comparison microscope
  to compare bullets and cartridge cases
• Explain the concept of the NIBIN database.

3
Chapter Objectives

• Explain the procedure for determining how far a
  weapon was fired from a target
• Indentify the laboratory tests for determining
  whether an individual has fired a weapon
• Explain the forensic significance of class and
  individual characteristics to the comparison of
  tool mark, footwear, and tire impressions
• List some common field reagents used to enhance
  bloody footprints.

4

• Minute, random markings on surfaces can impart
  individuality to inanimate objects
• Scratches, nicks, breaks, and general wear on a
  bullet
• Scratch or abrasion marks on a tool
• General wear on a tire
• Firearms identification
• A discipline mainly concerned with determining
  whether a bullet or cartridge was fire by a
  particular weapon
• Requires knowledge of operation of all types of
  weapons, restoration of obliterated serial
  numbers, detection and characterization of
  gunpowder residues

5
Bullet and Cartridge Comparisons

• The inner surface of the barrel of a gun leaves
  its markings on a bullet passing through it
• Markings are peculiar to each gun
• If one bullet is found at the scene and another
  is test-fired from a suspects gun, and they show
  the same markings, the suspect is linked to the
  crime.

6
The Gun Barrel

• Gun barrel is produced from a solid bar of steel
  that has been hollowed out by drilling
• Drill leaves marks on barrels inner surface
• Barrel also has spiral grooves which are the
  low-lying portions between the lands in a rifled
  bore
• Grooves contribute to rifling, the spinning of
  the bullet as it leaves the barrel

7
The Gun Barrel
The surfaces of the original bore remaining
between the grooves are called lands
8
The Gun Barrel

• Spinning bullet improves the accuracy of the shot
• The caliber of a firearm is the measurement of
  the space between opposite lands (diameter of
  barrel, measuring from lands, not grooves)
• Commonly measured in hundredths of an inch or
  millimeters.

9
Rifling Methods C/I Characteristics

• Every firearms manufacturer chooses a rifling
  process that is best suited to meet the
  production standards and requirements of its
  product
• Once this choice is made, all bullets made by
  this manufacturer will have the same class
  characteristics (same number of grooves and
  lands)
• Minute striations created by wear on the
  manufacturing equipment impart individual
  characteristics on each barrel.

10
Comparing Bullet Markings

• As the bullet passes through the barrel, its
  surface is impressed with the rifled markings of
  the barrel

11
Comparing Bullet Markings

• Examiners must obtain test bullets fired through
  the suspect barrel for comparison
• Differences in the number of lands and grooves,
  and their direction of twist are the easiest ways
  to eliminate the possibility of the suspect
  bullet and test bullet traveling through the same
  barrel
• If they both have the same class characteristics,
  the suspect and test bullet is then passed on to
  a specialized analyst for further comparisons
  with a comparison microscope.

12
Comparison Microscope
13
Considerations in Bullet Comparison

• Firearms examiners rarely find a perfect match
  all around the bullet
• Grit and rust can alter the markings on bullets
  fired through the same barrel
• The force of impact distorts the bullet so often
  only a small area is left with in tact markings
• Striations in the barrel are not permanent
  structures-they change with wear
• Final opinion must be based on the judgment,
  experience and knowledge of the expert.

14
Considerations in Bullet Comparison

• FE may receive a spent bullet with no
  accompanying suspect weapon, and have to
  determine the caliber and make of the weapon
• Class characteristics that can help in
  identification
• Number of lands and grooves
• Direction of the twist
• Widths of lands and grooves.

15
Considerations in Bullet Comparison

• The FBI maintains a record known as the General
  Rifling Characteristics File
• Contains listings of class characteristics of
  different makes and models of firearms
• Periodically updated and disbursed to law
  enforcement community.

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Considerations in Bullet Comparison

• Shot guns
• Smooth barrel, no rifling
• Also, no bullets ? they shoot small lead balls or
  pellets
• FE must work with the shell casings to gather
  information (i.e. gauge of the barrel).

17
Cartridge Cases

• The act of pulling a trigger releases the
  weapons firing pin, causing it to strike the
  primer, which in turn ignites the powder
• The expanding gases generated by the burning
  gunpowder propel the bullet forward through the
  barrel, simultaneously pushing the spent
  cartridge case or shell back with equal force
  against the breechblock.

18
Cartridge Cases

• Like the bullet, the shell also gets its own
  markings by its contact with the inside of the
  barrel
• Shape of the firing pin
• Surface markings of the breechblock
• Extractor and ejector mechanisms
• Magazine clip (if present)
• Chamber walls.

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Key Points

• The manufacture of a gun barrel requires
  impressing its inner surface with spiral grooves,
  a step known as rifling. Rifling imparts spin to
  the projectile when it is fired, which keeps it
  on an accurate course.
• No two rifled barrels have identical striation
  markings. These striations form the individual
  characteristics of the barrel. The inner surface
  of the barrel of a gun leaves its striation
  markings on a bullet passing through it.

20
Key Points

• The class characteristics of a rifled barrel
  include the number of lands and grooves and the
  width and direction of twist
• The comparison microscope is a firearms
  examiners most important tool because it allows
  two bullets to be observed and compared
  simultaneously.

21
Key Points

• The firing pin , breechblock, and ejector and
  extractor mechanism also offer a highly
  distinctive signature for individualization of
  cartridge cases
• Unlike handguns, a shotgun is not rifledit has a
  smooth barrel. Because of this, shotgun shells
  are not impressed with any characteristic
  markings that can be used to compare two shotgun
  shells to determine whether they were fired from
  the same weapon.

22
Automated Firearms Search Systems

• Because of the price of many firearms, and their
  increased use in various crimes, many times a
  single firearm is used in multiple crimes
• Storing information on bullets and cartridge
  cases involved with crimes on a shared network
  allows this information to be retrieved from
  several jurisdictions.

23
Early Systems

• First national computerized database was started
  in the 1990s by two competed groups FBI and the
  Bureau of Alcohol, Tobacco, Firearms and
  Explosives (ATF)
• FBI
• Known as DRUGFIRE
• Emphasized the examination of unique markings on
  the cartridge casings.

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Early Systems

• ATF
• IBIS (Integrated Ballistic Identification System)
• Processed digital microscopic images of
  indentifying features found on both expended
  bullets and cartridge casings
• Joining forces
• 1999 FBI and ATF joined forces to create NIBIN
  (National Integrated Ballistics Information
  Network)
• ATF is responsible for system sites FBI is
  responsible for communications network.

25
Early Systems

• NIBIN
• Contains files from bullets and cartridge cases
  retrieved from crime scenes or test fires from
  retrieved firearms
• Used by more than 200 law enforcement agencies
  worldwide
• Contains more than 800,000 images
• In the US, NIBIN has been used to connect more
  than 11,000 bullets and casings to more than one
  crime.

26
Early Systems

• NIBIN serves only as a screening tool for
  firearms evidence
• The final comparison will be made by a forensic
  examiner through traditional microscopic methods.

27
Ballistic Fingerprinting

• Conceptual project to move to a more detailed
  system than NIBIN
• Would involve the capture and storage of
  appropriate markings on bullets and cartridges
  test fired before they are sold to the public
• Many questions and issues involved before it can
  be put into practice
• Whose responsibility will it be to compile and
  distribute the files?
• Where will the funding come from? Etc.

28
Key Points

• The advent of computerized imaging technology has
  made possible the storage of bullet and cartridge
  surface characteristics in a manner analogous to
  automated fingerprint files
• Two automated firearms search systems are
  DRUGFIRE, developed by the FBI, and IBIS,
  developed by the ATF
• NIBIN is the National Integrated Ballistics
  Information Network, a unified firearms search
  system that incorporates both DRUGFIRE and IBIS
  technologies.

29
Gunpowder Residues

• Modern ammunition is propelled forward by the
  expanding gases created by the ignition of
  gunpowder
• Ideally, all of the powder is ignited and
  converted into the expanding gases for the most
  propulsion
• In reality, there are particles left over, either
  unburned or in the smoke, and could end up on the
  target (if close enough) or the shooter.

30
Distance Determination

• The process of determining the distance between
  the firearm and a target, usually based on the
  distribution of powder patters or the spread of a
  shot pattern
• These patterns can be used to support or refute
  pleas of self-defense, signals of foul play,
  potential suicides, etc
• If an investigator does not get a suspect weapon,
  the best he/she can do is state whether a shot
  could have been fired from a particular distance.

31
Handguns and Rifles

• The precise distance from which a handgun or
  rifle has been fired must be determined by
  carefully comparing the powder residue pattern on
  the victims clothing or skin to test patterns
  made when the suspect weapon is fired at varying
  distances from a target
• In the case, it is necessary to have the suspect
  weapon and suspect ammunition in custody to make
  test fires.

32
Handguns and Rifles

• Without the weapon, the examiner is restricted to
  looking for recognizable characteristics around
  the bullet hole
• At best, these are approximations based on the
  examiners experience.

33
Handguns and Rifles

• Notable characteristics of close proximity
• When the weapons is held in contact with or less
  than 1 inch from the target, a heavy
  concentration of smokelike vaporous lead usually
  surrounds the bullet entrance hole
• Scorch marks from explosion
• Blowback of muzzle gases may forma star-shape
  tear pattern in fabrics, also surrounded by a rim
  of smokelike vaporous lead.

34
Handguns and Rifles

• Other notable characteristics
• 12-18 inches a halo of smoke deposited around a
  bullet hole
• 25 inches presence of scattered specks of
  unburned and partially burned powder grains, but
  no soot
• 3 feet or more no powders deposited on target.
• All of these can be affected by barrel length,
  caliber, type of ammunition, and type and
  condition of the weapon fired.

35
Shotguns

• The determination of firing distances involving
  shotguns must also be related to test firing
  performed with the suspect weapons, using the
  same type of ammunition involved with the crime
• In the absence of a weapon, a muzzle-to-target
  distance can be estimated by measuring the spread
  of the discharged shot.

36
Shotguns

• Characteristics of distances
• Up to 5 feet shot charge enters the target as a
  concentrated mass, slightly larger than the bore
  of the barrel
• As the distance increases, the pellets
  progressively separate and spread out
• Approximately 1 inch for every yard of distance
  (12-gauge shotgun)
• May be affected by the choke
• An interior constriction placed at or near the
  muzzle end of a shotguns barrel to control shot
  dispersion.

37
Powder Residues on Garments

• Surfaces of all clothing involved in shooting are
  examined for gunpowder residues
• May be identifiable based on characteristic
  colors, sizes and shapes
• Use of microscopes, different kinds of light, etc
  may be used to make the particles more visible
• Greiss Test may be used to confirm that particles
  are in fact gunpowder residue (nitrates).

38
Key Points

• The distribution of gunpowder particles and other
  discharge residues around a bullet hole permits
  an assessment of the distance from which a
  handgun or rifle was fired
• The precise distance from which a handgun or
  rifle is determined by carefully comparing the
  powder residue pattern on the victims clothing
  to test patterns made when the suspect weapon is
  fired at varying distances from a target.

39
Key Points

• The Griess test is a chemical test used to
  develop patterns of gunpowder around bullet
  holes. It tests for the presence of nitrates.

40
Primer Residues on the Hands

• The firing of a weapon blows gunpowder and primer
  residues back toward the shooter
• Traces of these residues are often deposited on
  the firing hand of the shooter
• Shows whether an individual has recently fired a
  weapon.

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Detecting Primer Residues

• Dermal Nitrate Test
• Early test to detect the presence of nitrates on
  a shooters hand
• Applied hot paraffin or wax to suspects hand
• Wax then removed and color tested for nitrates
• Blue color indicated a positive result for the
  presence of nitrates
• Presence of fertilizer, cosmetics, urine and
  tobacco all tested positive as well.

42
Detecting Primer Residues

• Primer Residues
• Contain chemicals unique to firearms (lead
  styphnate, barium nitrate, and antimony sulfide)
• Found most commonly on the thumb web and back of
  the firing hand
• May also be present on the palm of the hand just
  from coming into contact with the weapon.

43
Tests for Primer Residues

• Determination of whether a person has fired or
  handled a weapon is normally made by measuring
  the presence and amount of barium and antimony on
  the suspects hands
• Most popular approach is to apply adhesive tape
  to the surface of the hand in order to remove any
  residue particles.

44
Swabbing

• Remove any residues present by swabbing both the
  firing and nonfiring hands with cotton that has
  been moistened with a 5 nitric acid solution
• 5 swabs (2 for each hand, one control) are then
  sent to the crime lab for analysis
• High barium and antimony levels strongly suggest
  that the person fired or handled a weapon.

45
SEM Testing

• SEM Scanning Electron Microscope
• Makes it possible to see microscopic primer and
  gunpowder particles removed with adhesive tape
• Characteristic size and shape of these particles
  distinguishes them from other contaminants on the
  suspects hands
• Maybe linked to an X-ray analyzer to determine
  the elemental nature of the particles.

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Other Primer Residue Tests

• Neutron activation analysis
• Flameless atomic absorption spectrophotometry
• Both are sensitive tests that could detect minute
  amounts of primer residue
• Not often used because the equipment is big,
  expensive and complicated to use.

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Key Points

• Firing a weapon propels residues toward the
  target and blows gunpowder and primer residues
  back toward the shooter
• Traces of these residues are often deposited on
  the firing hand of the shooter, providing
  valuable information as to whether an individual
  has recently fired a weapon.

49
Key Points

• Examiners measure the amount of barium and
  antimony on the relevant portion of the suspects
  hands or characterize the morphology of particles
  containing these elements to determine whether a
  person has fired or handled a weapon or was near
  a discharged firearm.

50
Serial Number Restoration

• Many manufactured items, including automobile
  engine blocks and firearms, are impressed with a
  serial number for identification
• Increasingly, forensic scientists must restore
  such a number when it has been removed or
  obliterated by grinding, rifling or punching.

51
Serial Number Restoration

• Serial numbers are usually stamped on a metal
  body or frame, or on a plate, with hard steel
  dies
• Serial numbers can be restored because the metal
  crystals in the stamped zone are placed under a
  permanent strain that extends beneath the
  original numbers
• When an etching agent is applied, the strained
  area dissolves faster than the regular metal.

52
Collection and Preservation of Firearms Evidence

• Firearms
• Picking up a weapon by its barrel with a pencil
  or stick (like on tv) disturbs evidence on the
  barrel (powder deposits, rust, dirt)
• Should be picked up by the trigger guard or the
  checkered portion of the grip
• Next, the weapon must be unloaded to avoid
  accidental firing
• Location of all fired and unfired ammunition in
  the weapon must be recorded.

53
Collection and Preservation of Firearms Evidence

• Firearms
• Must be marked for identification and a chain of
  custody must be established
• When a weapon is recovered from an underwater
  location, no effort must be made to dry or clean
  it
• Should be transported to the laboratory in a
  receptacle containing enough of the same water
  necessary to keep it submerged
• Prevents rust from developing during transport.

54
Collection and Preservation of Firearms Evidence

• Ammunition
• Protection of class and individual markings on
  bullets and cartridge casings is primary concern
• If the bullets surface is accidentally scratched
  while being removed from a wall or body, valuable
  striations could be obliterated
• Surrounding material should be removed from
  around the bullet, instead of pulling the bullet
  out first.

55
Collection and Preservation of Firearms Evidence

• Ammunition
• Bullet should then be wrapped in tissue paper and
  put in a labeled pillbox or envelope
• Gunpowder Deposits
• Clothing of a firearms victim must be preserved
  to prevent damage or disruption of powder
  residues
• Clothing should not be cut or torn
• All wet clothing must be air dried out of direct
  sunlight
• Each item should be placed in a separate paper
  bag.

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Key Points

• Criminalists can restore serial numbers removed
  or obliterated by grinding, rifling or punching
• Because the metal crystals in the stamped zone
  are placed under a permanent strain that extends
  a short distance beneath the original numbers,
  the serial number can be restored through
  chemical etching
• A suspect firearm should never be picked up by
  inserting an object into its barrel because this
  practice may alter the striations.

57
Key Points

• Protection of class and individual markings on
  bullets and cartridge cases is the primary
  concern of the field investigator when recovering
  bullets and cartridge casings.

58
Tool Marks

• A tool mark is any impression, cut, gouge, or
  abrasion caused by a tool coming into contact
  with another object
• Often encountered at burglary scene involving
  forcible entry into a building or safe
• Occur as indented impressions into a softer
  surface or as abrasion marks caused by the tool
  cutting or sliding against another object.

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Comparing Tool Marks

• Examination can reveal class characteristics (ex.
  Size and shape of tool)
• Edges of different tools can display microscopic
  irregularities that make them unique to one
  particular tool, but difficult to discern
• Easiest to examine with soft surfaces (caulk,
  putty, etc)
• Difficult to replicate the angle and pressure of
  the tool used in a lab setting.

61
Collecting Tool Mark Evidence

• Whenever practical, the entire object or the part
  of the object bearing a tool mark should be
  submitted
• When its impossible to do so, the only other
  option currently available is a photograph of the
  marked area to scale and make a cast of the mark
• http//www.crime-scene-investigator.net/csi-video.
  html

62
Collecting Tool Mark Evidence

• Crime-scene investigator must never attempt to
  fit the suspect tool into the tool mark
• The suspect tool and mark must be packaged in
  separate containers, with every precaution taken
  to avoid contact between the tool or mark and
  another hard surface
• The tool or its impression may contain valuable
  trace evidence (ex. Chips of paint, fibers from
  clothing).

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Key Points

• The presence of minute imperfections on a tool
  imparts individuality to that tool
• The shape and pattern of such imperfections are
  further modified by damage and wear during the
  life of the tool
• The comparison microscope is used to compare
  crime-scene tool marks with test impressions made
  with the suspect tool.

64
Other Impressions

• Other impressions could be left at a crime scene
  (ex. Shoe, tire, fabric)
• Preserving impressions
• Primary consideration preservation of the
  impression or its reproduction (making a cast)
• Before its moved or handled, it must be
  photographed
• Ideally, the impression is transported whole to
  the lab
• If not possible, a cast is made.

65
Lifting Impressions

• If an impression is encountered on a surface that
  cannot be submitted, it may be able to be
  lifted like a fingerprint
• Often used with light deposits of dust or dirt
• Electrostatic lifting
• Sheet of mylar film is placed over the dust mark,
  and the film is pressed against the film with a
  roller
• Static electricity binds the particles to the
  film

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Casting Impressions

• Shoe and tire marks impressed into soft earth at
  a crime scene are best preserved by photography
  and casting
• Gypsym is widely recommended for making casts of
  shoe and tire impressions
• http//www.crime-scene-investigator.net/csi-video.
  html

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Comparing Impressions

• Comparison is only possible when an item
  suspected of having made the impression is
  recovered
• Test impressions are then made to compare
  characteristics
• New computer software may help compare shoe
  prints (SICAR Shoeprint Image Capture and
  Retrieval).

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Key Points

• Shoe and tire marks impressed into soft earth at
  a crime scene are best preserved by photography
  and casting
• The electrostatic lifting technique is
  particularly helpful in recovering barely visible
  dust prints on floor surfaces.