Basic Operation
• When the trigger is pulled to fire the weapon, the
hammer is driven down onto the firing pin, which in
turn strikes the primer of the cartridge.
• The primer mixture explodes, and this event in turn
causes ignition of the powder.
• The powder burns rapidly, generating an enormous
amount of gas pressure within the confined space
of the cartridge.
• The pressure forces the slug out of the cartridge,
through the barrel, and out of the muzzle of the
weapon.
• Each of these steps leaves class and individualistic
characteristics on the bullets and casings
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Basic Operation
Video compliments of www.firearmsid.com and Hi-Point firearms
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The Analysis
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Class Characteristics GRC
• General Rifling Characteristics are the four class characteristics
1. Caliber (bore diameter: .357, .22, .45)
2. Number of lands and grooves
3. Width of the lands and grooves
4. Direction of twist
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What’s your Caliber
• The diameter of a gun barrel, measured between
opposite lands, is known as the caliber of the
weapon.
• In general, caliber denotes the nominal diameter of
a barrel.
• In the United States and other English-speaking
countries, the caliber is measured in hundredths or
thousandths of an inch, for example .22, .38, .45,
.220, .357, or .405 caliber.
• In other countries, the caliber may be designated in
millimeters, for example, 6.5, 8, or 9 mm.
• The designation 7.65 mm is equivalent to .30 or .32
caliber (depending on the means of measurement),
whereas 9 mm is equivalent to .38 caliber.
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Measuring Caliber
• The caliber is the bore
diameter of the
firearm.
• The caliber is
measured at the base
of the bullet
• Calipers are read by
using the scale on the
side.
• This bullet measures
– 0 inches +
– 4 tenths +
– 51 thousandths =
– 0.451 inches
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Counting the Lands and Grooves
The grooves of the bullet are
counted
It may be necessary to place a
small mark with a felt tip pen in
the location where counting
begins.
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2
3
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Width of the Lands and Grooves
• Calipers are used to measure the width of the bullet lands
and grooves
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Direction of Twist
• The twist is determined by sitting the bullet on a
flat surface and observing the direction of the
rifling.
Left
Right
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Class Characteristics-Measured
• All class characteristics
are measurable
•Caliber
•Number of lands and grooves
•Width of the lands and grooves
1
2
3
•Direction of twist
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The Next Step
• Once we establish the agreement of the class
characteristics, we evaluate the individual
characteristics
• If we look beyond the lands and grooves, we can also see
microscopic scratches called striations.
• These microscopic scratches are a by-product of the
manufacturing process and are totally unique to a particular
firearm.
• Thus, the scratch marks on the bullet match the scratch
marks in the barrel of the gun that fired it, to the exclusion of
all other weapons.
• The matching of scratch marks is tested in the lab by test
firing a bullet through the suspect weapon.
• The test fired bullet is then compared under a comparison
microscope, side by side, to the bullet recovered from the
crime scene.
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•Both bullets are rotated until the striations can be made to line up, showing a
match.
•If the striations cannot be lined up…the result is negative
Real Striation Comparison
under a microscope
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Cartridge Case Comparison
• When a weapon is fired, markings can also be
impressed and scratched onto the cartridge
casing by various parts of the gun's
mechanism.
• As with striation marks on bullets, these
markings will be reproduced on a test
cartridge fired by the same gun.
• A firearms examiner can compare markings
on cartridges and link a cartridge case to a
specific weapon.
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Cartridge Case Comparisons
• The most characteristic marks expected on
a fired cartridge case are
– Headstamp
– Breech-Face Marks
– Firing Pin Indentations
– Extractor Marks
– Ejector Marks
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Headstamp
• Cartridge cases may be stamped at the factory to indicate
make and type.
• These stamped markings, sometimes referred to as
cartridge designations, or HEAD STAMPS can be used to
determine the manufacturer of the ammunition.
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Firearm Nomenclature
Breechface
Ejector
Firing Pin Aperture
Extractor
© Precision Forensic Testing
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Parallel Breechface Marks
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Rectangular Firing Pin Marks
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Extractor Marks
• Virtually all modern repeating firearms have some form of
extractor or ejector, although ejectors are not used in
revolvers.
• The extractor is a small part sometimes resembling a hook
that is used to remove a cartridge or cartridge case from the
chamber of a firearm.
• As the slide of the pistol moves to the rear, the extractor pulls
the cartridge case along with it until it is ejected from the
pistol.
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Extractor Comparison
• The marks made by the
contact of the extractor
with the rim of the
cartridge case can be
used for comparison
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Ejector Marks
• The ejector is designed
to eject the fired
cartridge case from the
firearm
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Ejector Marks
• The ejector comes in contact with the cartridge as
the slide moves rearward. When the ejector hits
the cartridge, it is “kicked” from the slide and out
of the firearm.
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Ejector Marks
• The contact between
the cartridge case and
the ejector results in
an ejector mark
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