Ballistics 101

1
Ballistics 101

• Just the facts, the very basic facts

2
Ballistics

• Just like fingerprints having INDIVIDUAL
  CHARACTERISTICS- so do bullets
• They get individual char. created on the SURFACE
  once they are fired from a gun
• STRUCTURAL VARIATIONS AND IRREGULARITEIS CAUSED
  BY SCRATCHES, NICKS, BREAKS AND WEAR ARE
  INDIVIDUAL MARKERS FOR BOTH FIREARMS AND OTHER
  TOOLMARKS- SHOE PRINTS, SHARP OBJECTS.

3
BULLET COMPARISON

• The inner surface of the barrel of a gun leaves
  markings on a bullet as it passes through the
  barrel
• each are individual to each gun.
• therefore, if a bullet is found at a crime scene,
  it can be compared to a test fire of the
  suspects weapon and matched

4
Individual Markings

• Gun barrel is produced from a solid bar of steel
  hollowed out by drilling.
• Microscopic drill marks are left on inner surface
  and randomly irregular- giving uniqueness to the
  barrel.

5
Rifling

• Manufacture of a barrel requires impressing the
  INNER SURFACE with
• 1)spiral grooves process known as rifling.
• 2) leaves the original surface of the BORE raised
  between the grooves called LANDS.
• As a bullet travels through the barrel- rifling
  grooves cause bullet to rapidly spin for straight
  projection

6
Caliber

• Diameter of the gun barrel is measured between
  opposite lands- known as the CALIBER of the
  weapon- usually recorded as hundredths of an inch
  or millimeters. (.22, .38, and 9mm)

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Methods for Rifling

• Cutting tool is rotated as it passes down the
  barrel- either to the right or left. I
• Manufacture still uses this process for mass
  production.
• 1)broach cutter- shaft with concentric rings
  each larger than the other, as it passes through
  the barrel, it cuts grooves to the appropriate
  depth to give it DESIRED DIRECTION AND RATE OF
  TWIST.
• 2) BUTTON process involves no cuttings, instead a
  steel plug or button impresses number of
  grooves and passes under extreme pressure-
  COMPRESSING THE METAL TO CREATE LANDS AND
  GROOVES. Rotated to produce desired direction
  and rate of twist.
• 3) mandrel rifling- no cutting- rod of machined
  steel is inserted into an oversized bore and
  barrel is compressed into the mandrels form.

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Class Charcteristics

• Types of manufacture is unique to each company
  and they maintain consistent production of type
  used this only produces class characteristics of
  the type of gun.
• Smith Wesson revolvers have 5 lands and grooves
  twisting to the right.
• Colt .32 caliber revolvers have 6 lands and
  grooves twisting to the left.
• These are still class characteristics, but help
  narrow down type or brand of weapon

9
Individual Characteristics

• Closer examination of barrel shows STRIATIONS
  running the length of the barrel's lands and
  grooves.
• THESE ARE PRODUCED FROM IMPERFECTIONS OF THE
  RIFILING PROCESS
• These are impossible to duplicate, even if
  manufactured in succession they will not have
  identical striation markings.
• As the bullet passes through barrel- the surface
  is impressed with rifled markings of the barrel.

10
Making the Comparison

• Examiner must compare striations to the suspect
  weapon in order to compare characteristics
• THE NUMBER OF LANDS AND GROOVES AND THEIR
  DIRECTIONAL TWIST ARE OBVIOUS POINTS OF
  COMPARISON DURING THE INITIAL STAGES OF
  EXAMINATION
• A bullet with 5 lands and grooves could not have
  been fired by a gun with 6, likewise a right
  twist could not have come from a left twist
  barrel

11
Individual Comparison

• Once its determined both bullet has same class
  characteristics, then placed under comparative
  microscope.
• This is the single most important tool of the
  firearms examiner.
• Test and evidence bullets are mounted on
  cylindrical adjustable holders and rotated under
  the microscope.
• Once a well-defined land or groove comes into
  view- then other bullet is rotated until a
  matching region is found

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Individual Comparison

• Lands and grooves must have IDENTICAL WIDTHS
  (Class)
• LONGITUDINAL STRIATIONS ON EACH MUST COINCIDE..
• once a matching area is identified, the two
  bullets are rotated to get additional matching
  points.
• Rare to get a perfect match completely around the
  bullets periphery.
• Presence of grit, rust can alter markings fired
  through the same barrel.
• Recovered bullets may be extremely deformed from
  impact, preserving only a small are for
  comparison.

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• Striation markings of a barrel change or alter
  from wear from succeeding bullets but not
  dramatic enough to obscure pattern.
• ???what if there is no comparison bullet from the
  suspects weapon???
• Weapons expert must provide information about
  caliber and possible make of the weapon.
• The number of lands and grooves and direction of
  twists are useful class char.
• Sometimes a bullet will have rifling marks unique
  to manufacture such as Marlin rifles these
  weapons are rifled by a technique known as
  microgrooving and may have 8-24 grooves impressed
  in the barrel.

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Ballistics Database

• FBI maintains records known as the general
  rifling characteristic file contains listings of
  class char---- lands and grooves width
  dimensions, and direction of twist.
• This is distributed to law enforcement and
  firearm examiners to aid in identification of
  rifled weapons from retrieved bullets.

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Smooth Barrels

• SHOT GUNS HAVE SMOOTH BARRELS- no markings to
  relate it to the weapon fired.
• Shotguns use small pellets or lead balls within a
  shotgun shell- paper or plastic wad pushes
  pellets through the barrel UPON IGNITION OF THE
  CARTRIDGES POWDER CHARGE.
• Measure and weigh diameter of shot recovered at
  scene reveals the GAGUE of the shotgun.

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GAUGE

• Diameter of the shotgun barrel is expressed as
  the GAUGE.
• The higher the gauge number, the smaller the
  barrels diameter
• 12 gauge shotgun has a bore diameter of .73
  inches as compared to .67 for a 16 gauge

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• CARTRIDGE CASES-
• The act of pulling a trigger releases the
  weapons firing pin causing it to strike the
  primer
• This ignites the powder
• Expanding gasses generated by burning gun powder
  propel the bullet forward through the barrel
  while simultaneously pushing the spent cartridge
  case or shell back with equal force against the
  breechblock

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Individual Characteristics

• As the Bullet is marked through the passage of
  the barrel the shell is impressed with markings
  by its contact with the metal surfaces of the
  firing and loading mechanisms
• Shape of the firing pin will be impressed in to
  the metal of the primer of the cartridge case,
  leaving distortions of firing pin
• These can individualize the pin impression to a
  single weapon

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Individualization

• Other distinguishing markings that may appear on
  the shell result from metal on metal contact
  caused by the ejector and extractor mechanisms on
  the magazine clip.
• Firing pin, breechblock, extractor, and ejector
  marks may also be impressed onto the surface of
  the brass portion of the shells fired by shotgun.
  
• These are valuable points to individualizing
  SHELL TO WEAPON.

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Needle in a Haystack

• AUTOMATED FIREARM SEARCH SYSTEMS database
  storage of bullet and cartridge surface
  characteristics- like fingerprints.
• Crime labs can be networked, sharing info on
  bullets and cartridges retrieved at several
  jurisdictions.
• Automated search system developed for the FBI-
  called DRUGFIRE- system holds unique markings of
  cartridge casings expended by the weapon.
• Image is captured by video camera, digitized and
  stored in database.

21
IBIS

• IBIS- Integrated Ballistic Identification
  System- for Bureau of Alcohol, Tobacco and
  Firearms
• uses digital microscopic images found both on
  expended bullets and cartridges.
• Both systems serve as screening tools and require
  skills of firearm investigator to make direct
  comparisons for positive ID.

22
GSR

• GSR- GUN POWDER RESIDUES
• Incidents involving wounds- must determine
  distance from which the bullet was fired.
• Often the accused will plead self-defense as
  motive for the attack. NECESSARY FOR DISTANCE
  DETERMINATION to find proximity of the parties
  during incident.
• Wounds of suicide victims reveals characteristics
  associated with very close-range wound- absence
  of such is strong indication that wound was NOT
  SELF INFLICTED.
• Ammunition is propelled by expanding gasses
• Created by ignition of smokeless powder or
  nitrocellulose in cartridge
• Powder is never totally burned
• When discharged, unburned and partially unburned
  gasses and smoke are propelled out of the barrel
  with bullet
• Distribution of gunpowder particles and discharge
  residues around the bullet hole permits
  assessment of distance from which handgun was
  fired.
• When you cant recover the weapon, must determine
  distance.
• Comparison of powder residue on clothes or skin
  against controlled test patterns to get distance
  estimates for type of gun.

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Distance Characteristics

• Recognizable characteristics around bbullet hole
• Held in contact concentration of smokelike vaporous lead
  surrounding hole----clothing will have scorch
  markes from flame discharge-----blowback of
  muzzle gasses produce stellate (star-shaped) tear
  patterns around the hole and surrounded by a rim
  of smokelike vaporous lead.
• 12-18 inches or less presence of stipple or
  scattered specks from unburned and partially
  burned powder. Grains without any soot can be
  seen up to 25 inches away.
• Scattered powder can be seen as far as 36 inches.
• After 3 ft, target wil not have any powder on
  surface- only mark will bee a dark right known as
  BULLET WIPE around of perimeter of entrance hole-
  mixture of carbon, dirt, llubricant, primer
  residue, and lead wiped off of bullets surface
  as it passes
• All used for estimates of range---- factors
  such as barrel length, caliber, type of
  ammunition and type and condition of weapon all
  effect GSR on target.

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Primer Residues

• Primer residues firing a weapon not only propels
  residues toward the target, but blown back onto
  the shooter. These are deposited on the firing
  hand- determines that person has recently fired a
  weapon.
• Chemical tests used to detect GSR- dermal
  nitrate test- hot wax on the suspects hand to
  trap residue and tested with the chemical
  diphenylamine- turned blue as positive reaction
  with nitrates---however, gave false positives
  with fertilizers, cosmetics, urine, and tobacco.
• Primer residues include lead styphnate, barium
  nitrate and antimony sulfide- tested on palms,
  fingers and back of hands.
• To test for GSR use swab of cotton with 5 nitric
  acid. Front and back of each hand swabbed
  separately with 4 areas, b\plus a 5th control
  area (under shirt or protected area). This tests
  for barium and antimony rather than just
  nitrates.
• Can have GSR if firing a weapon, if near someone
  firing a weapon, touch a surface near a fired
  weapon Horrible method- GSR can be washed
  off, rubbed off, and deteriorates from hands
  after 2 hours.
• Cannot detect in .22 rim fire ammunition since
  does not contain barium or antimony in its primer
  composition.