Ch. 17 Ballistics PP Notes

Report
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:
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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
Ballistics is the study of bullets and firearms.
Ballistic evidence helps explain:
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What type of firearm was used
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The caliber of the bullet
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The number of bullets fired
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Where the shooter was located
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Whether a weapon was fired recently
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If a firearm was used in previous crimes
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Forensic Science: Fundamentals & Investigations, Chapter 17
History of
Gunpowder and Firearms
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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
muzzle
Modern firearms are divided into 2 basic
groups:
Long Guns and Handguns
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Long guns
•
•
•
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Handguns
Pistols are fired with one hand
• Revolvers have a cylinder (that holds usually six cartridges)
that turns as it is fired
• The Semiautomatic is loaded with up to 10 cartridges into a
magazine (clip); fire only 1 bullet per pull of the trigger
• The Fully Automatic will fire repeatedly as long as the
trigger is pressed
Forensic Science: Fundamentals & Investigations, Chapter 17
•
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Rifles fire bullets
Shotguns fire pellets (shot) or a single projectile (slug)
Both require the use of 2 hands for accurate firing.
Pictures of Firearms
Shotgun
Rifle
Revolver
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Semiautomatic
Forensic Science: Fundamentals & Investigations, Chapter 17
Fully Automatic
Firearms and Rifling
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o
o
o
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Grooves are indentations in the
rifle’s barrel. The ridges (raised
areas) that surround the grooves
are called lands.
Grooves and lands in the barrel
of a gun produce the twisting that
adds accuracy.
The spiral pattern of lands and
grooves in the barrel of a firearm
is called rifling.
This leaves a pattern on the
bullet that matches the barrel
pattern to a specific firearm.
Forensic Science: Fundamentals & Investigations, Chapter 17
Bullets, Cartridges, and Calibers
o
o
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.
headstamp
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Forensic Science: Fundamentals & Investigations, Chapter 17
How a Firearm Works
1. Pull the trigger and
the firing pin hits the
base of the cartridge,
igniting the primer
powder mixture.
2. The primer powder sparks through the flash hole to
the main propellant (gunpowder) supply.
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Forensic Science: Fundamentals & Investigations, Chapter 17
How a Firearm Works
3. The main gunpowder
ignites, and the
pressure of the
explosion pushes the
bullet from the casing
into the barrel.
4. The bullet follows the lands and grooves pattern of
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the barrel and begins to spiral before it leaves the
barrel.
Forensic Science: Fundamentals & Investigations, Chapter 17
Caliber of the Cartridge
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Bullets (and their cartridges) are named by caliber and length.
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Caliber—a measure of the diameter of the cartridge—by
hundredths of an inch. (Europeans use the metric system-9 mm)
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Common calibers include:
.22, .25, .357, .38, .44, and .45
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A .45 caliber cartridge measures 45/100 of an inch in diameter
(almost ½ an inch).
o
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Caliber also refers to the diameter of the inside of a firearm’s
barrel; therefore, the caliber of ammunition should match the
firearm that shoots it.
If a bullet is removed from a wound or crime scene, its caliber
can link it to the weapon used to fire it.
Forensic Science: Fundamentals & Investigations, Chapter 17
The Study of Bullets
and Cartridge Casings
1.
How is each fired bullet marked? As a gun is fired, the barrel
marks each bullet with its own unique pattern of lands and grooves.
2. What is the procedure to match a spent bullet to the
firearm that shot it? Investigators compare bullets and spent
cartridges shot from the suspected firearm. To get a known bullet for
comparison, they test-fire the weapon.
3.
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What makes up a test-firing, and why is it done?
Investigators test-fire the weapon into a water tank or gel block. This
captures the bullet without damaging it. Then, they can compare the
markings on known bullets with those on the suspect bullets.
Marks on the
Spent Cartridge Casings
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Firing pin marks—impressions made on the bottom of
the cartridge by the firing pin as it strikes the bottom of the
cartridge
• Appear on the rim or center of the spent cartridge
• Can be used to match a cartridge to a firearm
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Breechblock marks--produced when the cartridge
casing slams backward and strikes the breechblock
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The markings are unique to the firearm
Can be matched if the spent cartridge casings are
found
Marks (cont.)
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Extractor marks--minute scratches produced as the
cartridge is placed in the firing chamber (by the extractor)
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Ejector marks--minute scratches produced as the
cartridge is removed from the chamber (by the ejector)
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These marks are produced only in semiautomatic
and fully automatic weapons
In revolvers, cartridges are hand-fed into the
revolving cylinder and have to be removed by
hand as well
Forensic Science: Fundamentals & Investigations, Chapter 17
Databases
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A searchable collection of information stored
in a computer system
Firearms databases can be searched to
match crime-scene evidence to registered
weapons
2 important databases:
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NIBIS—National Integrated Bullet Identification System (ballistic
markings of firearms used in previous crimes)
Drugfire—and FBI database (focuses on cartridge casings)
In 2000, these databases were merged to form NIBIN--National
Integrated Ballistics Network
How is NIBIS used to help solve
crimes?
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Law enforcement agents can use this
database to see if they can match a bullet
found at a crime scene to a firearm that was
used to commit a crime in the past.
If this connection is made, how could this
information help a law enforcement
agent?
Forensic Science: Fundamentals & Investigations, Chapter 17
Gunshot Residues
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Gunshot Residues (GSR)
• Particles of unburned powder and traces of smoke
• Traces are left on the hand, arm, face, hair, or
clothing of the shooter and/or victim (evidence)
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Chemical testing can detect residue even if
removal (washing) is attempted
Distance from victim to shooter can be
determined by examining the residue pattern on
the victim
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The amount of GSR decreases as the distance
between firearm and victim increases
Trajectory
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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
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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
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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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
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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
23 .9"
AB


So
or
Ac
AC
23 .5" 720 "
AB = 732.3”

Forensic Science: Fundamentals & Investigations, Chapter 17
Triangulation
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Using Pythagorean’s theorem
AB2 = AC2 + BC2
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732.32
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BC2 = 732.32 – 7202
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BC2 = 536117 – 518400
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BC = √17717 (square root)
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BC = 133.1 inches
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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 – Questions to
Discuss…
Examining body wounds can determine where a bullet
entered and exited the victim, which help determine
where the shooter was located by identifying the
bullet’s path (or trajectory).
1. Why do entrance wounds tend to be smaller than exit
2.
3.
4.
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5.
wounds?
If the bullet penetrates clothing, what can fibers embedded
in the wound indicate?
Where on the victim 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?
Answers
1.
2.
3.
4.
5.
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Generally, the size of the entry would will be smaller than the bullet
because skin in somewhat elastic (it stretches when a bullet enters
the body). Exit wounds are generally larger because as the bullet
moves through the body, it may collect and carry body tissue and
bone with it.
Fibers imbedded in wounds can indicate the direction of penetration.
Gunshot residue can be found only around entrance wounds.
If the gun is fired with the muzzle touching the victim’s skin, the hot
gases released from the muzzle flash may burn the skin, leaving a
telltale mark.
High-speed bullets are more likely to pass through a body than are
low-velocity bullets. Therefore, small-caliber bullets, such as a .22
caliber, tend to lodge within the body, while larger-caliber bullets will
pass through.
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

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