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Firearms & ToolmarksFirearms Identification A discipline
mainly concerned with determining whether a bullet or cartridge was
fired by a particular weapon. It is not to be confused with
ballistics, which is the study of a projectile in motion.
Bullet Comparison1)The barrel of a gun leaves markings on a
bullet fired through it that are particular to that
barrel.2)Suspect guns are test fired and the test bullet is
compared with the evidence bullet.
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Firearms & Toolmarks
Firearms Identification A gun barrel is produced fromA solid bar
of steel that has been hollowed out by a drill.
Rifling:The spiral grooves formed in the bore of a firearm
barrel that impart spin to the projectile when it is fired.
Grooves: The cut or low-lying portions between the lands in a
rifled barrel
Lands: The raised portion between the grooves in a rifle
bore.
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Firearms & Toolmarks
Rifling The act of making lands and grooves in the barrel of a
rifle which impart spin to the projectile fired through the
barrel.
Cutting: Prior to 1940 all barrels were machine cut by running a
steel hook cutter through the barrel which would impart one or two
grooves at a time.
2) Cutting: After 1940 a broach cutter was used so all grooves
could be cut simultaneously.3) Button Process: A steel plug or
button impressed with the desired number of grooves is forced under
extremely high pressures down the barrel and the inside of the
barrel is compressed to form the lands & grooves4) Mandrel
Rifling: A mandrel rod of hardened steel cut so its form is the
reverse of the desired lands & grooves is inserted into a
slightly oversized bore. The mandrel-containing bore is then
compressed so the lands & grooves are formed.
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Firearms & Toolmarks
As the bullet passes through the barrel, the barrel rifling and
other unique characteristics impress markings on the bullet which
are unique to that barrel.
No two rifled barrels, even those manufactured in succession,
have identical striation markings.Striation Markings
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Firearms & Toolmarks
Firearms Identification To compare bullets, an examiner test
fires the weapon into a recovery box filled with cotton or a water
tank.Bullet Comparison: What to look for1) Class Characteristicsa)
The number of lands & groovesb) Their direction of twist.2)
Individual Characteristicsa) Striated markings on each bulletb)
Done using a Comparison Microscope
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Firearms & Toolmarks
Bullet ComparisonDifficulties often arise during the attempt to
identify or compare bullets:
1) Grit & Rust in the barrel can alter the markings on
bullets fired through the same barrel2) Evidence bullet mutilation
commonly, the evidence bullet, when found is mutilated to some
degree due to its passage through materials (bone, tissue, walls,
car doors, etc)3) Extended weapon use as the weapon is fired over
& over, the striations on the barrel will change to some degree
over time. However, its usually not enough to totally eliminate the
match4) Examiner expertise unlike chemical or instrument analysis,
bullet comparison relies on the expertise and experience of the
examiner to determine whether two bullets are, in fact, a
match.
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Firearms & Toolmarks
An example of bullet mutilation & deformation due to
projectile velocity.
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Firearms & Toolmarks
Copper-washedlubaloyRound-nosedWad-cutterSemi-wadcutternycladTeflon-coatTHV
Solid brassmachined bulletJacketed soft-point
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Firearms & Toolmarks
Copper-jacketedNickel-platedGlaser Safety Slug containing small
lead pellets and a plastic plugFederal Expanding
Full-Metal-Jacketed (EFMJ)bullet with silicone nose material Fully
jacketed
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Firearms & Toolmarks
Speer Gold-Dot Jacketed Hollow-point bullet. Sabot Round
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Firearms & Toolmarks
Shotguns have smooth barrels so there is no rifling and no
subsequent striations on the projectile.Shotguns are measured by
bore size or gauge.12-gauge 0.73 in. (1/12th lb)16-gauge 0.67 in
(1/16th lb)20-gauge 0.53 in (1/20th lb)
410-gauge 0.41 in.
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Firearms & Toolmarks
Cartridge Case ComparisonThe act of pulling the trigger releases
the weapons firing pin, causing it to strike the primer, which in
turn ignites the gunpowder. The expanding gases generated by the
burning gunpowder propel the bullet forward through the barrel,
simultaneously pushing the spent cartridge case or shell back with
equal force against the breechblock (the rear part of a firearm
barrel).
As the bullet is marked by its passage through the barrel, the
shell casing or cartridge is marked by contact with the metal
surfaces of the weapons firing & loading mechanisms.
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Firearms & Toolmarks
Cartridge Case ComparisonAs with bullets, these markings are
distinctive.
The shape of the firing pin is impressed into the primer.
The breechblock characteristics are impressed into the rear of
the cartridge case.
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Firearms & Toolmarks
Cartridge Case Comparison
Firing Pin Impressions: these can be very unique and can be seen
in both center-fire and rim-fire cartridges.
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Firearms & Toolmarks
Cartridge Case Comparison
Breechmark Comparisons: Here is an example of the markings on
the back of the breech being impressed on the
casing.BreechblockCartridge Casing
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Firearms & Toolmarks
Cartridge Case Comparison
Ejector Marks: These are a function of the ejection mechanism as
it grabs the cartridge case and ejects it after firing.Ejector
MechanismEjector marks 1Ejector marks 2
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Firearms & Toolmarks
Cartridge Case Comparison
Shear Marks: These are made as the cartridge, after being
pressed back into the breechblock, is then moved up or down across
the firing pin hole and a set of ridges are cut into the
casing.
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Firearms & Toolmarks
Automated Firearms Identification DRUGFIRE (started by the FBI)
and IBIS (started by the ATF) were initially incompatible systems
in the early 1990s.Around 1998 2000 the FBI and ATF joined forces
and created NIBIN (National Integrated Ballistics Information
Network).
DRUGFIRE focused on examination of spent cartridge casings and
stored digital images. IBIS concentrated on expended bullets as
well as casings but the two computer storage systems couldnt be
linked (sorta like PC and Mac)
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Firearms & Toolmarks
Configuration of a portion of the IBIS system called
Bulletproof
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Firearms & Toolmarks
Gunpowder Residues Used to help determine the distance from
which the weapon was fired.
Distance Determinations: The process of determining the distance
between the firearm and a target, usually based on the distribution
of powder patterns or the spread of a shot pattern.
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Firearms & Toolmarks
Chemical evaluation of GSR can expand the evidence pattern to
include nitrate residue as well as lead and powder residue.
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Firearms & Toolmarks
Following the initial explosion, vaprous lead along with powder,
nitrates and other gases are expelled from the barrel
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Firearms & Toolmarks
Tool Marks
A tool mark is any impression, cut, gouge, or abrasion caused by
a tool coming into contact with another object. Most often tool
marks are encountered at burglary scenes or other forcible entry
scenes. Generally, these marks occur as indented impressions into a
softer surface or as abrasion marks caused by the tool cutting or
sliding against another object.
As with bullets and rifling, here we can have Class
Characteristics as well as Individual Characteristics.
Class characteristics include the type and shape of tool as well
as size. These would be the difference between a screwdriver and a
claw hammer or a chisel that was used to force an entry,
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Firearms & Toolmarks
Tool Marks
Individual characteristics include random striations generated
during the machining of the tool as well as individual edge
characteristics like nicks and cuts or ridges and valleys that
develop in tools over time.These individual characteristics can be
seen in all types of tools from chisels to shovels to cutters and
saws.
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Firearms & Toolmarks
Here both class and individual characteristics can be seenHere
the casting of a tool mark using silicone brings out the individual
characteristics of this prybar.