Firearms, Took Marks & Other Impressions

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Firearms, Took Marks Other Impressions

• Chapter 15
• Mr. Carl

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Firearms A Quick History

• Almost every gun is based on the same simple
  concept You apply explosive pressure behind a
  projectile to launch it down a barrel.
• The earliest simplest application of this idea
  is the cannon.

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The 1st handheld guns were essentially
mini-cannons you loaded some gunpowder a steel
ball lit a fuse.
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• War typically resulted in the need for improved
  weapons technology.
• In the late 1800s, the revolver quickly became
  popular due to its size quick loading.
• It only had to be reloaded every 5-6 shots
  instead of after each shot.

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• Handguns reigned supreme for the past 200 years
  to this day, remain the most popular readily
  available firearm.

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Firearms Identification

• Often confused with the term ballistics
• Ballistics is the study of a projectile in
  motion.
• Inside the firearm
• After it leaves the firearm
• When it impacts the target

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Firearms Identification

• Identification of Firearms is based upon this
  basic idea
• A harder object marks a softer one
  imparts/transfers its microscopic irregularities
  to that object.

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Forensic Firearms Expert

• Did a suspect use this gun to kill that person?
• Did these bullets come from that gun?
• Was it really self-defense?
• Is this a case of suicide, or is foul play
  involved?

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Forensic Firearms Expert

• Bullet Comparison
• Weapons Function
• Is it safe? Has it been modified?
• Serial Number Restoration
• Gunpowder Residue Detection
• on clothes, hands, wounds
• Muzzle-to-Target Distances

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Pulling the Trigger

• Pulling the trigger releases the firing pin
• The firing pin strikes the primer
• The primer ignites the gun powder
• The powder generates gas that propels the bullet
  forward through the barrel ejects the spent
  cartridge case.

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• Caliber the diameter of the gun barrel.
• Caliber is recorded in
• hundredths of an inch (.22 .38)
• millimeters (9mm)

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Bullet Anatomy
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Anatomy of a Bullet
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Bullet Comparisons

• Each gun leaves distinct markings on a bullet
  passing through it.
• A gun barrel is made from a solid bar of steel
  that has been drilled/hallowed out.
• The drill leaves microscopic marks on the
  barrels inner surface.

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Bullet Comparisons

• Gun manufacturers also add spiral grooves to the
  barrel. This is known as rifling.
• Lands the space between the grooves.
• As a spinning bullet passes through the barrel,
  it is marked by these grooves.

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Class Characteristics

• Class Characteristics Once a manufacturer
  chooses a rifling process, for a particular class
  of weapon, they keep it consistent.
• Lands Grooves are the same for a model.
• .32 caliber Smith Wesson has 5 lands grooves
  twisting to the right.
• .32 caliber Colt has 6 lands grooves twisting
  to the left.
• Class characteristics can eliminate certain makes
  but are not enough to ID a particular gun.

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Individual Characteristics

• Imperfections in the manufacturing process make
  each barrel unique.
• Rifled barrels, even if made in succession will
  NOT have identical striation (scratch-like
  marks).

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Bullet Comparisons

• To match bullets to a gun, test bullets must be
  fired through a suspect barrel for comparison.
• Goddard Comparison Microscopes
• Examined bullets side-by-side (to match striated
  markings).

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Bullet Comparisons
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Cartridge Markings

• All moving components contact the cartridge
  rather than the bullet can leave useful
  impressions on shell cartridges
• Breechface marks
• Firing pin impressions
• Chamber marks
• Extractor Ejector markings

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Breechface Marks
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Actual Breech Marks
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Firing Pin Marks
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Chamber Marks
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Ejector Marks
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Other Factors

• Perfect matches sometimes difficult b/c
• Presence of grit rust in a barrel
• Recovered bullets too mutilated or distorted on
  impact
• A spent bullets weight can sometimes determine
  the gun make.
• Microgrooves 8-24 grooves its not as common
• General Rifling Characteristics File
• FBI database of known land/groove width for all
  weapons.

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Shotguns

• Smooth barrel
• Projectile NOT marked as it passes through
• Fire small lead balls or pellets contained within
  a shell.
• Characterized by
• diameter of the shot
• size shape of the wad
• Gauge diameter of the barrel
• (? gauge ? diameter)

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Gunshot Residue (GSR)

• GSR Sources
• victim, clothing or target
• shooters hands
• Gunpowder Chemistry
• Major detectable elements are lead (Pb), barium
  (Ba) antimony (Sb)
• Virtually all cartridge cases are made of brass
  (copper zinc) also detectable.

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Griess Test

• Tests for the presence of nitrates (partially
  burned or unburned gunpowder)
• Swab of shooters hand
• Must produce a pattern for a distance
  determination

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Results of GSR Hand Test

• Negative results may be caused by
• Washing the hands
• Shooter may have been wearing gloves
• Lead free ammunition
• A rifle or shotgun may not deposit GSR on hands

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GSR on the hand of a suicide victim, proving he
was holding the weapon when it was fired.
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?DANGER? ? GRAPHIC IMAGES!
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Contact Gunshot wound

• This is a contact gunshot entrance wound.
• Since the barrel contacts the skin, the gases
  released by the fired round go into the
  subcutaneous tissue cause the star-shaped
  laceration.

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Abrasion Ring
The abrasion ring, and a very clear muzzle
imprint, are seen in this contact range gunshot
wound.
An abrasion ring, formed when the force of the
gases entering below the skin blow the skin
surface back against the muzzle of the gun, is
seen here in this contact range gunshot wound to
the right temple.
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Displayed here is an entrance at the left and an
exit at the right. Exit wounds vary considerably
in size and shape because the bullet can be
deformed in its transit through the body. There
may be no exit wound at all if the bullet's
energy is absorbed by the tissues. Some bullets
(such a a "hollowpoint") are designed to deform
so that all their energy will be converted to
tissue damage and not exit.
This is a contact range gunshot entrance wound
with grey-black discoloration from the burned
powder.
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• Powder tattooing is seen in this intermediate
  range gunshot wound. The actual entrance site is
  somewhat irregular, because the bullet can tumble
  in flight.

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• The surface of the skull demonstrates the heavy
  soot in this contact range entrance wound, as
  well as radiating fracture lines. The direction
  of fire was thus toward the back of this picture.

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• With a contact or very close range gunshot wound,
  it is possible to have blood spatter as well as
  GSR on the hand of the person firing the weapon.

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END GRAPHIC IMAGES!
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Trajectory
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DC Sniper
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SERIAL NUMBER RESTORATION

• When a serial number is stamped into a gun, the
  metal underneath the number is compressed
  hardened.
• If the number is filed-off, the hardened area may
  still be present.
• By using an acid solution the metal can be slowly
  eaten away.
• In this process the softer metal will be eaten
  away first and the number may reappear.

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Before After

• Historically, serial numbers are successfully
  restored 63 of the time on steel 54 of the
  time for aluminum or zinc.

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FIREARMS EVIDENCE COLLECTION

• Make sure it is unloaded!!!!!
• DO NOT put a pencil into a barrel
• REVOLVERS
• Indicate location of fired unfired ammunition
• AUTOMATICS
• Check magazine for number of rounds
• Fingerprint magazine
• Place ID tag on trigger guard

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FIREARMS EVIDENCE COLLECTION

• AMMUNITION
• Write on base or nose
• Package in pill box or envelope
• Wrap in tissue to protect

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FIREARMS EVIDENCE COLLECTION

• CLOTHING
• Protect preserve any residue
• Air dry if wet
• Package separately in paper bags
• Establish CHAIN OF CUSTODY

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TOOL MARK IMPRESSIONS

• Impressions
• Cuts
• Gouges
• Abrasions

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TOOL MARK IMPRESSIONS

• SUBMIT ENTIRE OBJECT
• PACKAGE SEPARATELY (ALWAYS!)
• DO NOT ATTEMPT TO FIT THE TOOL INTO THE TOOL MARK

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