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TRANSCRIPT
THE SCIENCE OF FIREARMS
IDENTIFICATION INCLUDES:
• Comparison of bullets and cartridge
casings
As well as:
• Knowledge of all types of weapons
• Restoration of obliterated serial numbers on
weapons
• Detection and characterizations of
gunpowder residue on garments / around
wounds / hands
• Estimation of muzzle-to-target distances
•There are literally thousands of types of firearms!
They can be classified as shoulder firearms (rifles
and shotguns) and handguns (pistols, revolvers,
etc)
•A gun barrel is produced from a solid bar of steel
by drilling. The manufacture of a barrel also requires
the impression of spiral grooves on the inner
surface. This is called “rifling”. The surfaces of the
original bore are called lands. The diameter of a
gun barrel, measured land to opposite land is
called the “caliber” of the weapon. It is measured in
hundredths of an inch or in millimeters.
•The lands and grooves guide the bullet through the
barrel, giving it spin, so that it does not tumble,
thus increasing its speed and force
• Every firearms manufacturer
chooses a rifling process, giving the
weapon class characteristics.
Frequently, a firearms examiner is
given a spent bullet without a
weapon. The caliber, weight, etc.
help determine the weapon. The
FBI maintains a record of class
characteristics known as the GRCF
(Gallery Rifle Centre Fire).
Full Metal Jacket vs Hollow Point
• Some bullets are described as full metal jackets (FMJ) and some are described as hollow points (HP).
– A hollow point is designed to stay inside the victim, as to not potentially exit the victim and strike someone else.
– These bullets are often used by police so that other spectators aren’t injured.
– A full metal jacket is designed to pass through the victim.
•The marks or striations on
the inner surface of the barrel
are randomly irregular and
make each unique. As the
bullet passes through the
barrel, its surface is
impressed with unique rifled
marks of the barrel.
•There are small changes
due to grit, dust, & wear.
•Recovered evidence bullets may become so
distorted on impact as to yield only a small area
with markings.
9mm LUGER caliber G.F.L.
(Fiocchi) FMJ bullets.
38 SPECIAL caliber plain lead bullet.
32 S&W LONG caliber
plain lead bullet.
22 caliber copper-washed and
plain lead bullets.
A comparison microscope is used to
compare striation marks. No rules govern
the points needed for bullet comparison.
A bullet is slightly larger in diameter than the bore diameter of the barrel in which it is designed to be fired. The bore diameter is the distance from one land to the opposite land in a barrel. As a result, a rifled barrel will impress a negative impression of itself on the sides of the bullet like those seen below.
Pulling the trigger serves
to release the weapon’s
firing pin, causing it to
strike the primer, which
ignites the powder.
The burning gunpowder
generates gases that
expand and propel the
bullet forward, and the
spent cartridge case
back against the breech
block.
The shell therefore also
has unique markings!
Land Impression Striations
380 AUTO caliber FMJ bullets
fired from a EAA Corp. pistol.
9mm LUGER caliber FMJ bullets
fired from a BROWNING pistol.
Groove Impression Striations
38 SPECIAL caliber
lead bullets
fired from a ROHM
revolver.
38 SPECIAL caliber
SJHP bullets
fired from a TAURUS
revolver.
A shotgun shell is diagrammed below:
Gauge: Refers to the
diameter of the barrel on
a shotgun in terms of the
number of lead balls the
size of the bore it would
take to weigh one pound
(10 gauge, 12 gauge,
etc.) ".410 gauge" really
refers to caliber, but is
worded as such to refer
to a shotgun.
Bullets, shot pellets,
and other projectiles
may be recovered
from the crime scene
or from the bodies of
shooting victims. Fired
cartridge cases, fired
shot shells, shot shell
wadding, unfired
ammunition and
components may also
have evidentiary
value.
Matching breech marks Breech marks do not match
Firing pin impressions
38 SUPER AUTO cartridge
cases fired in a COLT pistol.
Gunshot Residue
The deposition of gunshot
residue on evidence such
as clothing varies with the
distance from the muzzle
of the firearm to the target.
Patterns of gunshot
residue can be duplicated
using a questioned firearm
and ammunition
combination fired into test
materials at known
distances. These patterns
serve as a basis for
estimating muzzle-to-
garment distances.
Gunshot residue patterns.
T-shirt chemically treated for the presence of lead residue.
Heavy deposits of lead found along left shoulder.
Close-up of bullet entrance hole in above shirt.
Lead bullet wipe residues (pink) noted around the hole.
9-inch test standard showing visible particulate residue (lead, burned and
unburned gunpowder) and vaporous lead (pink) residues.
3-inch test standard
showing a very
concentrated
deposit of soot and
vaporous lead
residues.
Griess Test from the 3-inch test
standard. Orange color is a
positive reaction to a presence
of a pattern of nitrite residues.
Soot is seen on the hand of a suicide victim,
giving an indication that he was holding the
weapon when it was fired.
Collection and Preservation of
Firearms Evidence
• Never stick an object into the barrel
• Pick up the gun by the edge of the trigger guard or on the checkered portion of the grip – these areas do not usually retain prints
• Weapons recovered in water must be transported to the lab in the same water to prevent rusting in transit
• Ammunition must never be dug out of a wall or other surface – remove the whole section surrounding the bullet in order to reduce the possibility of adding any stray scratches, etc.