Firearms Notes
Forensic Science
Categories of Firearms
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Shotguns- gauge
Rifles-caliber
Handguns-caliber
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Rifles and handguns are rifled (have
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spiraled grooves built into barrel)
Gauge
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Gauge - the system used to determine
the interior diameter of the barrel of a
shotgun.
If you take one pound of lead and divide
that pound into 16 balls of equal size, the
diameter of those balls will fit a 16 gauge
shotgun. (it is the same inner diameter as
a 16 gauge shotgun)
Caliber
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Caliber - the system used to determine
the bore diameter of a handgun or rifle –
generally a measurement in either mm or
inches.
Bore diameter - the distance from land
to land in the inside of a rifled barrel.
Lands – the higher areas on the inside
surface of the barrel
Lands and Grooves
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Cut or pressed, the
entire length of a
barrel, into the inner
surface of a barrel in
a twisting motion.
Lower areas are
grooves
Higher areas are
lands
Rifling
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Rifling - a “cut”
made down the
center of the barrel
of a rifle.
Purpose - The “cut”
is made in a twisted
pattern to cause the
bullet to spiral as it
moves through the
air.
Rifling Cont’d
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A rifle’s “Twist” is the barrel distance for one
spiral to be completed. Can vary from 8 to 20
inches.
How much twist a gun has depends on the type
of ammunition to be used (mass of bullet and
amount and type of powder)
The number of grooves inside the barrel can
also vary. From 3 to 8 grooves
For example, rifle can have a 4 groove 14 twist.
Anatomy of a Firearm
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Breechblock
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Supports base of the cartridge in
the chamber
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Often finished by hand filing
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characteristic striations on this
part contact the back of the
cartridge leaving individualizing
markings on the cartridge base
Anatomy of a Firearm
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Firing Pin
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Strikes the cartridge primer
to initiate the firing process
Finished on a lathe or filing
by hand
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Can transfer striations to
the soft metal of the
primer cap
Firearm Anatomy, Cont.
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Extractor and Ejector
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extracts and ejectors a spent
cartridge from the chamber
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Metal parts of extractor and
ejector can leave
individualizing markings on
cartridge cases
All of the moving components
contact the cartridge rather than
the bullet and can leave useful
impressions on shotgun shell
cartridges
Ammunition
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A cartridge
(ammunition) is made
up of gun powder, a
primary explosive and
a bullet all held within
a cartridge case
bullet
powder
Cartridge
case
Primer cup
What Happens When the Trigger is
Pulled?
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1. Pulling the trigger causes the firing pin to
strike the back of the primer cup, leaving the
impression of the firing pin on the back of the
primer cup. The primer cup contains a primary
explosive. (A primary explosive is one that is
sensitive to friction, heat and or mechanical
shock.)
2. When the primary explosive receives this
mechanical shock it explodes.
Pulling the Trigger Cont’d
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3. This explosion causes the gun powder in the
cartridge case to ignite. The burning of gun
powder (KNO3 + sulfur + carbon) produces the
gases CO2, nitrogen oxides and SO2.
4. The gases, produced in this reaction, take up
greater space causing the bullet that is in front
of them to be forced forward through the rifled
barrel of the gun. Rifling marks and striations
from the inside surface of the barrel are
impressed upon the sides of the bullet.
Action and Reaction
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For every action there is an equal and opposite
reaction.
When the bullet is pushed forward, it pushes
back on the cartridge case and the gun itself
(recoil).
The cartridge case is pushed back against the
breechface (rear wall of the firing chamber) of
the gun. The striations found on the breechface
are impressed upon the back of the cartridge
case.
Removing the Cartridge Case
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The cartridge case is then removed from the
firing chamber by an extractor. The extractor, a
metal device, leaves a mark on the edge of the
cartridge case rim.
The ejector then pushes the cartridge case out
of the firearm, leaving its mark on the side of
the cartridge case. These marks, made by the
extractor and ejector, will match the marks
made on other cartridge cases fired by that
same firearm.
Class vs. Individual
Class vs. Individual
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Breechface marks, firing pin marks and the
striations found on the sides of the bullet have
individual characterisitics.
Rifling marks on the bullet only show the
number of grooves and information that can
give the twist.
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This is class information only.
It can sometimes tell the manufacturer of a firearm.