Criminalistics: An Introduction to Forensic
Science
Chapter 9
Firearms, Tool Marks, and
Other Impressions
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Firearms
• Firearms identification is a discipline mainly concerned
with determining whether a bullet or cartridge was fired
by a particular weapon.
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Handguns (1 of 2)
• Single-shot pistols
– Fire only one round at a time
• Revolvers
– Feature several firing chambers within a revolving
cylinder
– Swing-out revolvers
– Break-top revolvers
– Solid frame revolvers
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Handguns (2 of 2)
• Semi-automatic pistols
– Feature a removable magazine
– Fire one shot per trigger pull
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Figure 9–1: A Swing-Out Revolver Features a
Cylinder That Swings out to the Side of the Weapon to
Be Loaded
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Figure 9–2: A Semiautomatic Pistol
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Long Guns (1 of 2)
• Long guns may be single-shot, repeating, semiautomatic, or automatic.
– Shotguns
▪ Shotgun ammunition (shell) contains numerous
ball-shaped projectiles called slug
▪ Narrowing of the smooth barrel (the choke of the
shotgun) can concentrate shot when fired
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Long Guns (2 of 2)
– Rifles
▪ Feature a barrel with lands and grooves
▪ Bullet ammunition is impressed with lands and
grooves during firing
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Figure 9–3: A Bolt-Action Long Gun Uses the
Movement of a Bolt Mechanism to Expel the Spent
Cartridge Case, Load the Next Round, and Cock the
Hammer
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Figure 9–4: A Semiautomatic Long Gun Uses the
Energy from the Firing Reaction to Expel the Spent
Cartridge Case, Load the Next Round, and Cock the
Hammer
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Gun Barrel Markings (1 of 5)
• The inner surface of the barrel of a gun leaves its
markings on a bullet passing through it.
• These markings are unique to each gun.
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Gun Barrel Markings (2 of 5)
• The gun barrel is produced from a solid bar of steel that
has been hollowed out by drilling.
• The microscopic drill marks left on the barrel’s inner
surface are randomly irregular and serve to impart a
uniqueness to each barrel.
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Gun Barrel Markings (3 of 5)
• The manufacture of a barrel also requires impressing its
inner surface with spiral grooves, a step known as rifling.
• Rifling is usually accomplished by:
– Cutting all grooves in one pass with a cutter known as
a broach, or
– Pressing all the grooves at once onto the barrel with a
tool known as a button, or
– Hummer forging the barrel using a mandrel containing
the reverse image of the rifling.
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Gun Barrel Markings (4 of 5)
• The surfaces of the original bore remaining between the
grooves are called lands.
• The grooves serve to guide a fired bullet through the
barrel, imparting a rapid spin to ensure accuracy.
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Gun Barrel Markings (5 of 5)
• The diameter of the gun barrel, measured between
opposite lands, is known as caliber.
• Once a manufacturer chooses a rifling process, the class
characteristics of the weapon’s barrel will remain
consistent.
– Each will have the same number of lands and
grooves, with the same approximate width and
direction of twist.
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Figure 9–5: Interior View of a Gun Barrel,
Showing the Presence of Lands and Grooves
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Striations
• Striations are fine lines found in the interior of the barrel.
– These striations form the individual characteristics of
the barrel.
• It is the inner surface of the barrel of a gun that leaves its
striation markings on a bullet passing through it.
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Figure 9–9: A Bullet is Impressed with the Rifling
Markings of the Barrel When It Emerges from the
Weapon
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Figure 9–11: Photomicrograph of Two Bullets
through a Comparison Microscope. The Test Bullet
is on the Right; the Questioned Bullet is on the Left
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Bullet Examination (1 of 2)
• No two rifled barrels will have identical striation markings.
• The number of lands and grooves and their direction of
twist are obvious points of comparison during the initial
stages of an examination between an evidence bullet and
a test-fired bullet.
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Figure 9–10: A Bullet Holder Beneath the
Objective Lens of a Comparison Microscope
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Bullet Examination (2 of 2)
• Any differences in these class characteristics immediately
serve to eliminate the possibility that both bullets traveled
through the same barrel.
• Individualization, a goal of in all areas of criminalistics,
frequently becomes an attainable reality in firearm
examination by examining the striations on a bullet.
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Shotguns
• Unlike rifled firearms, a shotgun has a smooth barrel.
• Shotguns generally fire small lead balls or pellets that are
not impressed with any characteristic markings that can
be related back to the weapon.
• The diameter of the shotgun barrel is expressed by the
term gauge.
• The higher the gauge number, the smaller the barrel’s
diameter.
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Firing a Weapon (1 of 2)
• Pulling the trigger releases the weapon’s firing pin,
causing it to strike the primer, which in turn ignites the
powder.
• 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 breechface.
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Firing a Weapon (2 of 2)
• The shell is impressed with markings by its contact with
the metal surfaces of the weapon’s firing and loading
mechanisms.
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Cartridge Case Comparison
• The firing pin, breechface, and ejector and extractor
mechanism also offer a highly distinctive signature for
individualization of cartridge cases.
• The shape of the firing pin will be impressed into the
relatively soft metal of the primer on the cartridge case.
• The cartridge case, in its rearward thrust, is impressed
with the surface markings of the breechface.
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Computer Imaging (1 of 2)
• Computerized imaging technology makes it possible to
store bullet and cartridge surface characteristics in a
manner analogous to automated fingerprint files.
• The National Integrated Ballistics Information Network (NI
BIN) produces database files from bullets and cartridge
casings retrieved from crime scenes or test fires from
retrieved firearms, often linking a specific weapon to
multiple crimes.
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Computer Imaging (2 of 2)
• The ultimate decision for making a final comparison will
be determined by the forensic examiner through
traditional microscopic methods.
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Distance Determination (1 of 3)
• When a firearm is discharged, unburned and partially
burned particles of gunpowder, in addition to smoke, are
propelled out of the barrel along with the bullet toward the
target.
• If the muzzle of the weapon is sufficiently close, these
products will be deposited onto the target.
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Distance Determination (2 of 3)
• The distribution of gunpowder particles and other
discharge residues around a bullet hole permits an
assessment of the distance from which a handgun or rifle
was fired.
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Distance Determination (3 of 3)
• The precise distance from which a handgun or rifle has
been fired is determined by carefully comparing the
powder-residue pattern located on the victim’s clothing or
skin against test patterns made when the suspect
weapon is fired at varying distances from a target.
• By comparing the test and evidence patterns, the
examiner may find enough similarity in shape and density
upon which to base an opinion as to the distance from
which the shot was fired.
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Distance Determination without a Suspect
Weapon (1 of 2)
• In cases where the weapon is held in contact with or less
than 1 inch from the target, a star-shaped (stellate) tear
pattern around the bullet hole entrance, surrounded by a
rim of a smokeless deposit of vaporous lead is usually
present.
• A halo of vaporous lead (smoke) deposited around a
bullet hole is normally indicative of a discharge of 12 to
18 inches or less.
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Distance Determination without a Suspect
Weapon (2 of 2)
• The presence of scattered specks of unburned and
partially burned powder grains without any accompanying
soot is often observed at distances up to 25 inches (and
occasionally as far as 36 inches).
• A weapon fired more than 3 feet from a surface/individual
will usually not deposit any powder residues, and the only
visual indication is a dark ring around the hole, known as
a bullet wipe.
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Figure 9–16a: Test Powder Patterns Made with a .38
Special Smith & Wesson Revolver Fired at the
Following Distances from the Target: Contact
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Figure 9–16b: Test Powder Patterns Made with a .38
Special Smith & Wesson Revolver Fired at the
Following Distances from the Target: 6 Inches
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Figure 9–16c: Test Powder Patterns Made with a .38
Special Smith & Wesson Revolver Fired at the
Following Distances from the Target: 12 Inches
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Figure 9–16d: Test Powder Patterns Made with a .38
Special Smith & Wesson Revolver Fired at the
Following Distances from the Target: 18 Inches
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Figure 9–17: A Contact Shot
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Figure 9–18: (a) A Shirt Bearing a Powder Stain,
Photographed under Normal Light. (b) an Infrared
Photograph of the Same Shirt
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Gunpowder Residue (1 of 2)
• Distance determination involving shotguns must again be
related to the suspect weapon and ammunition.
• In the absence of a weapon, muzzle-to-target distance
can be estimated by measuring the spread of the
discharged shot.
– As the shot distance increases, the pellets
progressively separate and spread out.
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Gunpowder Residue (2 of 2)
• When garments or other evidence relevant to a shooting
are received in the crime laboratory, the surfaces of all
items are first examined microscopically for the presence
of gunpowder residue.
• Chemical tests, such as the Greiss test, may be needed
to detect gunpowder residues that are not visible.
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Primer Residue on Hands (1 of 2)
• Firing a weapon propels residues toward the target, and
blows gunpowder and primer residues back toward the
shooter.
• As a result, traces of these residues are often deposited
on the firing hand of the shooter, and their detection can
provide valuable information as to whether or not an
individual has recently fired a weapon.
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Primer Residue on Hands (2 of 2)
• Examiners measure the amount of barium and antimony
on the relevant portion of the suspect’s hands, such as
the thumb web, the back of the hand, and the palm.
• Scientists may also characterize the morphology of
particles containing these elements to determine whether
or not a person has fired, handled a weapon, or was near
a discharged firearm.
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Figure 9–20a: Adhesive Stubs Used to
Sample a Suspect’s Shooter’s Hands
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Figure 9–20b: Sampling a Suspect’s Hand
for Gunshot Residue with an Adhesive Stub
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Serial Numbers
• Increasingly, the criminalist is requested to restore a
serial number when it has been removed or obliterated by
grinding, rifling, or punching.
• Restoring serial numbers is possible through chemical
etching because the metal crystals in the stamped zone
are placed under a permanent strain that extends a short
distance beneath the original numbers.
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Firearm Evidence Collection (1 of 3)
• Firearms are collected by holding the weapon by the
edge of the trigger guard or by the checkered portions of
the grip.
• Before being sent to the laboratory, all precautions must
be taken to prevent accidental discharge of a loaded
weapon.
• In most cases, it will be necessary to unload the weapon.
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Firearm Evidence Collection (2 of 3)
• When a revolver is recovered, the chambers, their
positions, and corresponding cartridges must be
recorded.
• Firearm evidence must be marked for identification
(usually a tag on the trigger guard) and a chain of
custody must be established.
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Firearm Evidence Collection (3 of 3)
• Bullets, cartridge cases, and shotgun shells recovered at
the crime scene must be packaged in a properly labeled
evidence container.
• Take care to avoid obliterating striation markings that may
be present on the bullet.
• Protect the bullet by wrapping it in tissue paper before
placing it in a pillbox or an envelope for shipment to the
crime laboratory.
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Tool Marks (1 of 3)
• A tool mark is any impression, cut, gouge, or abrasion
caused by a tool coming into contact with another object.
– Examining the impression can reveal important class
characteristics, such as the size and shape of the
tool.
– But it is the presence of any minute imperfections on
a tool that imparts individuality to that tool.
– The shape and pattern of such imperfections are
further modified by damage and wear during the tool’s
life.
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Figure 9-24: A Comparison of a Tool Mark
with a Suspect Screwdriver
Note how the presence of nicks and breaks on the tool’s
edge helps individualize the tool to the mark.
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Tool Marks (2 of 3)
• The comparison microscope is used to compare crimescene tool marks with test impressions made with the
suspect tool.
• When practical, the entire object or the part of the object
bearing the tool mark should be submitted to the crime
laboratory for examination.
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Tool Marks (3 of 3)
• Under no circumstances must the crime scene
investigator attempt to fit the suspect tool into the tool
mark.
– Any contact between the tool and the marked surface
may alter the mark and will, at the least, raise serious
questions about the integrity of the evidence.
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Other Impressions (1 of 3)
• Impressions of other kinds, such as shoe, tire, or fabric
impressions, may be important evidence.
• Before any impression is moved or otherwise handled, it
must be photographed (including a scale) to show all the
observable details of the impression.
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Other Impressions (2 of 3)
• If the impression is on a readily recoverable item, such as
glass, paper, or floor tile, transport the evidence to the
laboratory intact.
• If the surface cannot be submitted to the laboratory, the
investigator may be able to preserve the print in a
manner similar to lifting a fingerprint.
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Other Impressions (3 of 3)
• When shoe and tire marks are present at a crime scene,
their preservation is best accomplished by photography
and casting.
• In areas where a bloody footwear impression is very faint
or where the subject has tracked through blood leaving a
trail of bloody impressions, chemical enhancement can
visualize latent or nearly invisible blood impressions.
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Points of Comparison (1 of 2)
• A sufficient number of points of comparison or the
uniqueness of such points will support a finding that both
the questioned and test impressions originated from one
and only one source.
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Points of Comparison (2 of 2)
• Computer software and websites may be able to assist in
making shoe print and tire impression comparisons.
• Also, bite mark impressions on skin and foodstuffs have
proven to be important evidence in a number of homicide
and rape cases.
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FIREARMS
After an attempted home robbery, you are called to the crime scene. You must
interpret the events of the crime. You notice a security camera that has been
disabled by gunfire. In addition to the damaged security camera, you find a bullet
hole in the wall under the camera. In this experiment, you will analyze bullet
trajectory to determine the approximate shoulder height of the suspect.
Equipment:
Ruler
Procedure:
1. Use a ruler to measure the major and minor axis of the bullet hole below (page
3). Measure to the nearest millimeter.
2. Use the equation sin ϴ =
𝑚𝑖𝑛𝑜𝑟 𝑎𝑐𝑖𝑠 (𝑚𝑚)
𝑚𝑎𝑗𝑜𝑟 𝑎𝑥𝑖𝑠 (𝑚𝑚)
to determine the angle at which the
bullet impacted the wall. Record the angle in Table 1.
3. You find that the bullet hole is located 80" from the floor.
4. As the investigation proceeds, a latent shoe print is recovered 45" from the
wall with the bullet hole facing the wall. Draw a diagram of the wall, floor,
and bullet trajectory.
5. Use trigonometry to calculate the approximate shoulder height of the shooter.
Record the approximate shoulder height in Table 1. If you are unsure how to do
this, watch the instructional video.
Table 1: Bullet Trajectory Data
Post-Lab Questions
1. Record your calculations from Step 2 here.
2. Draw the diagram from Step 4 here.
3. Record your calculations from Step 5 here.
4. What are the winding grooves which help a bullet maintain a stable trajectory called?
5. Would you expect to find more gunshot residue around an entrance or exit hole? Explain
your reasoning.
6. Would a hand gun or shot gun be more difficult to trace and determine class
characteristics? Explain your answer.
7. The victim has a bullet hole that has a halo of soot surrounding the entrance hole along
with scattered specks of powder grains. What do you estimate the distance from the
shooter to the victim to be?
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