firearms, took marks & firearms: a quick history other ......1 firearms, took marks & other...
TRANSCRIPT
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Firearms, Took Marks &
Other Impressions
Chapter 15
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.
The 1st handheld guns were essentially
mini-cannons; you loaded some
gunpowder & a steel ball & lit a fuse.
• War typically resulted in the need for
improved weapons technology.
• In the late 1800’s, the revolver quickly
became popular due to it’s size & quick
loading.
– It only had to be reloaded every 5-6 shots
instead of after each shot.
• 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
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.
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?
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
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)
Bullet Anatomy 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
barrel’s inner surface.
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.
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.
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).
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).
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
Breechface Marks
Actual Breech Marks Firing Pin Marks
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Chamber MarksEjector Marks
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 bullet’s weight can sometimes determine the gun make.
• Microgrooves: 8-24 grooves; it’s not as common
• General Rifling Characteristics File
– FBI database of known land/groove width for all weapons.
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)
Gunshot Residue (GSR)
• GSR Sources:
– victim, clothing or target
– shooter’s 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.
Griess Test
• Tests for the presence of nitrates (partially
burned or unburned gunpowder)
• Swab of shooter’s 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
GSR on the hand of a suicide victim, proving
he was holding the weapon when it was fired.
Trajectory
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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.
Before & After
– Historically, serial numbers are successfully
restored 63% of the time on steel & 54% of
the time for aluminum or zinc.
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
FIREARMS EVIDENCE
COLLECTION
• AMMUNITION
– Write on base or nose
– Package in pill box or envelope
– Wrap in tissue to protect
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
TOOL MARK IMPRESSIONS
•SUBMIT ENTIRE OBJECT
•PACKAGE SEPARATELY (ALWAYS!)
•DO NOT ATTEMPT TO FIT THE TOOL
INTO THE TOOL MARK
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