Chapter 17 Ballistics
By the end of this chapter you will be able to:
o
Explain the differences between a handgun, a
rifle, and a shotgun
o
Describe rifling on a gun barrel and how it
affects the flight of the projectile
o
Explain barrel size and caliber
All Rights Reserved South-Western / Cengage Learning © 2012, 2009
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Forensic Science: Fundamentals & Investigations, Chapter 17
Chapter 17 Ballistics
By the end of this chapter you will be able to:
2
o
Describe how bullets are test fired and matched
o
Discuss the role of ballistics recovery and
examination at a crime scene
o
Determine the position of the shooter based on
bullet trajectory
Forensic Science: Fundamentals & Investigations, Chapter 17
Introduction
Ballistic evidence helps explain:
o
What type of firearm was used
o
The caliber of the bullet
o
The number of bullets fired
o
Where the shooter was located
o
Whether a weapon was fired recently
o
If a firearm was used in previous crimes
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Forensic Science: Fundamentals & Investigations, Chapter 17
History of
Gunpowder and Firearms
o
o
o
o
4
Chinese invented
gunpowder over a
thousand years ago
Muzzle-loading
matchlocks used wicks
to ignite the gunpowder
Cartridge and breech
loading
Revolver, semiautomatic, and
automatic handguns
Forensic Science: Fundamentals & Investigations, Chapter 17
Long Guns and Handguns
o
Long guns
•
•
o
Handguns
•
•
o
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Rifles fire bullets
Shotguns fire pellets (shot) or a single projectile
(slug)
Pistols are fired with one hand
Revolvers have a cylinder that holds usually six
cartridges
Automatic and Semi-automatic
Forensic Science: Fundamentals & Investigations, Chapter 17
Firearms and Rifling
o
o
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Grooves and ridges
(lands) in the barrel of a
gun produce the twisting
that adds accuracy
This leaves a pattern on
the bullet that is unique
Forensic Science: Fundamentals & Investigations, Chapter 17
Bullets, Cartridges, and Calibers
o
o
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Cartridge—a case that holds
a bullet, primer powder, and
gunpowder
The bullet, usually of metal, is
out front with the cartridge,
holding the primer and
propellant powders, behind.
Forensic Science: Fundamentals & Investigations, Chapter 17
How a Firearm Works
1. The firing pin hits the
base of the cartridge,
igniting the primer
powder
2. The primer powder sparks through the flash hole to
the main propellant supply
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Forensic Science: Fundamentals & Investigations, Chapter 17
How a Firearm Works
3. The pressure of the
explosion pushes the
bullet from the casing
into the barrel
4. The bullet follows the lands and grooves spiraling out
of the barrel
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Forensic Science: Fundamentals & Investigations, Chapter 17
Caliber of the Cartridge
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o
Caliber—a measure of the diameter of the
cartridge
o
In hundredths of an inch
o
Common calibers include
.22, .25, .357, .38, .44, and .45
o
Why should the caliber of ammunition
match the firearm that shoots it? If they do
not match, what could go wrong?
Forensic Science: Fundamentals & Investigations, Chapter 17
The Study of Bullets
and Cartridge Casings
How is each fired bullet marked?
2. What is the procedure to match a spent bullet to
the firearm that shot it?
3. What makes up a test-firing, and why is it done?
1.
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Forensic Science: Fundamentals & Investigations, Chapter 17
Marks on the
Spent Cartridge Casings
o
Firing pin marks
• appear on the rim or center of the spent cartridge
• can be used to match a cartridge to a firearm
o
Breechblock marks
• produced when the cartridge casing slams
backward and strikes the breechblock
o
o
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Extractor marks
Ejector marks
Forensic Science: Fundamentals & Investigations, Chapter 17
Gunshot Residues
o
Gunshot Residues (GSR)
• Particles of unburned powder and traces of smoke
• Leave traces on the hand, arm, face, hair, or clothing
of the shooter and/or victim
o
o
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Chemical testing can detect residue even if
removal is attempted
Distance from victim to shooter can be
determined by examining the residue pattern on
the victim
Forensic Science: Fundamentals & Investigations, Chapter 17
Trajectory
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o
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Two reference points are needed to define the
trajectory
Investigators can
figure the shooter
discharged the
firearm somewhere
along that line
Forensic Science: Fundamentals & Investigations, Chapter 17
Trajectory
o
Reference points can be
•
bullet holes in objects or victims
• An entry point and exit point on a victim
• Gunshot residue or spent cartridge casings
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Lasers can trace a straight-line path to
determine the position of the shooter
Forensic Science: Fundamentals & Investigations, Chapter 17
Trajectory and Gravity
o
o
Bullet’s path is
slightly curved
Gravity pulls it
downward as the
bullet moves forward
Diagram is highly exaggerated
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Forensic Science: Fundamentals & Investigations, Chapter 17
Determining the
Location of the Shooter
Building is 60 feet away along the horizon line
Bullet hole is 4 feet above the ground
Where is the
shooter
located?
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Forensic Science: Fundamentals & Investigations, Chapter 17
Triangulation
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o

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B is where the shooter is located; find the length of BC
The Abc triangle has the same proportions as the ABC
triangle
Ab AB

Ac AC
o
So
o
AB = 732.3”
or

Forensic Science: Fundamentals & Investigations, Chapter 17
23.9" AB

23.5" 720"
Triangulation
o
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Using Pythagorean’s theorem
AB2 = AC2 + BC2
o
732.32
o
BC2 = 732.32 – 7202
o
BC2 = 536117 – 518400
o
BC = √17717 (square root)
o
BC = 133.1 inches
o
BC = 11.1 feet
=
7202
+
BC2
Forensic Science: Fundamentals & Investigations, Chapter 17
We know that the
bullet hole in the
seat is four feet
above the ground,
so the shooter is
15.1 feet above the
ground
Bullet Wounds
1. Why do entrance wounds tend to be smaller than exit
2.
3.
4.
5.
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wounds?
If the bullet penetrates clothing, what can fibers
embedded in the wound indicate?
Where is gunshot residue usually found?
If the gun is fired with the muzzle touching the victim’s
skin, what telltale mark may show up?
Will larger or will smaller caliber bullets tend to lodge
within the body rather than passing through? Why?
Forensic Science: Fundamentals & Investigations, Chapter 17
. . . . . . . . . . . . . Summary . . . .
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Ballistics is the study of bullets and firearms.
Firearms are divided into two groups—long guns
and hand guns.
Fired bullets show patterns of lands and grooves
that match the rifling pattern in the barrel.
A cartridge consists of primer powder, gunpowder,
a bullet and the casing material.
The caliber of a cartridge usually is a measure of
its diameter.
Forensic Science: Fundamentals & Investigations, Chapter 17
. . . . . . . . . . . . . . . . . Summary
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Investigators also check for firing pin, breechblock,
extractor, and ejector marks.
Gunshot residue can help recreate a crime.
Using at least two reference points, an investigator
can recreate a bullets trajectory and determine
where the shooter was located.
Examining body wounds can determine where a
bullet entered and exited the victim.
Forensic Science: Fundamentals & Investigations, Chapter 17