chapter 17 ballistics - ms....
TRANSCRIPT
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Forensic Science: Fundamentals & Investigations, Chapter 17 1
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 2
Chapter 17 Ballistics By the end of this chapter you will be able to:
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
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Forensic Science: Fundamentals & Investigations, Chapter 17 3
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 4
History of Gunpowder and Firearms
o Chinese invented gunpowder over a thousand years ago
o Muzzle-loading matchlocks used wicks to ignite the gunpowder
o Flintlock weapons used a spark from a chip of flint
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Percussion firing weapons
o Cartridge – holds bullet, primer powder, gunpowder
o Hammer hits primer powder which ignites the gunpowder
o Breech loading is faster than muzzle loading.
Forensic Science: Fundamentals & Investigations, Chapter 17 5
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6
Long Guns and Handguns
o Long guns • Rifles fire bullets • Shotguns fire pellets (shot) or a single projectile (slug)
o Handguns • Pistols are fired with one hand • Revolvers have a cylinder that holds usually six
cartridges o Semi-automatic – 10 cartridges into a
magazine/clip. Fires one bullet per pull of trigger o Automatic – fires repeatedly as trigger is pulled
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Forensic Science: Fundamentals & Investigations, Chapter 17 7
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Forensic Science: Fundamentals & Investigations, Chapter 17 8
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Forensic Science: Fundamentals & Investigations, Chapter 17 9
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Forensic Science: Fundamentals & Investigations, Chapter 17 10
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Forensic Science: Fundamentals & Investigations, Chapter 17 11
Firearms and Rifling
o Grooves and ridges (lands) in the barrel of a gun produce the twisting that adds accuracy
o This leaves a pattern on the bullet that is unique
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Forensic Science: Fundamentals & Investigations, Chapter 17 12
Bullets, Cartridges, and Calibers
o Cartridge—a case that holds a bullet, primer powder, and gunpowder
o The bullet, usually of metal, is out front with the cartridge, holding the primer and propellant powders, behind.
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Forensic Science: Fundamentals & Investigations, Chapter 17 13
How a Firearm Works
2. The primer powder sparks through the flash hole to the main propellant supply
1. The firing pin hits the base of the cartridge, igniting the primer powder
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Forensic Science: Fundamentals & Investigations, Chapter 17 14
How a Firearm Works
4. The bullet follows the lands and grooves spiraling out of the barrel
3. The pressure of the explosion pushes the bullet from the casing into the barrel
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Forensic Science: Fundamentals & Investigations, Chapter 17 15
Caliber of the Cartridge
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?
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Forensic Science: Fundamentals & Investigations, Chapter 17 16
The Study of Bullets and Cartridge Casings
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Matching bullets
o Gun is “test-fired” into gel or water. o Doesn’t damage bullet o Markings are compared to suspect’s
weapons, bullets, casings.
Forensic Science: Fundamentals & Investigations, Chapter 17 17
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Forensic Science: Fundamentals & Investigations, Chapter 17 18
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 Extractor marks and Ejector marks
• Only in semi-auto and automatic weapons • Small scratches
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Forensic Science: Fundamentals & Investigations, Chapter 17 19
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Forensic Science: Fundamentals & Investigations, Chapter 17 20
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Forensic Science: Fundamentals & Investigations, Chapter 17 21
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Forensic Science: Fundamentals & Investigations, Chapter 17 22
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Forensic Science: Fundamentals & Investigations, Chapter 17 23
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Ejector marks
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Forensic Science: Fundamentals & Investigations, Chapter 17 25
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 Chemical testing can detect residue even if
removal is attempted o Distance from victim to shooter can be
determined by examining the residue pattern on the victim
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Forensic Science: Fundamentals & Investigations, Chapter 17 26
Bullet Wounds
o Can show what happened during the crime • Was the victim running away?
o Entrance wounds are smaller than exit wounds • Skin stretches as bullet enters • As bullet moves through the body it collects tissue
o Fibers may point in the direction the bullet moved o GSR found near entrance wounds o Muzzle may burn skin if gun was close o Small caliber bullets tend to remain lodged in the body
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Forensic Science: Fundamentals & Investigations, Chapter 17 27
Trajectory
o Two reference points are needed to define the trajectory
o Investigators can figure the shooter discharged the firearm somewhere along that line
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Forensic Science: Fundamentals & Investigations, Chapter 17 28
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
o Lasers can trace a straight-line path to determine the position of the shooter
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Forensic Science: Fundamentals & Investigations, Chapter 17 29
Trajectory and Gravity
o Bullet’s path is slightly curved
o Gravity pulls it downward as the bullet moves forward
o Wind can affect trajectory
o Bullet can hit other objects and ricochet Diagram is highly exaggerated
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Forensic Science: Fundamentals & Investigations, Chapter 17 30
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 31
Triangulation
o B is where the shooter is located; find the length of BC o The Abc triangle has the same proportions as the ABC
triangle
o So or
o AB = 732.3”
AbAc
=ABAC
23.9"23.5"
=AB
720"
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Forensic Science: Fundamentals & Investigations, Chapter 17 32
Triangulation
o Using Pythagorean’s theorem AB2 = AC2 + BC2
o 732.32 = 7202 + BC2
o BC2 = 732.32 – 7202
o BC2 = 536117 – 518400 o BC = √17717 (square root)
o BC = 133.1 inches o BC = 11.1 feet
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
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Activity 17-1
o To find the angle of elevation, measure the angle of the two reference points, then subtract from 90
o Measure the distance from the bullet to the building
Forensic Science: Fundamentals & Investigations, Chapter 17 33
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Activity 17-1
o Use Law of Tangents to find the height of the shooter above the bullet
Forensic Science: Fundamentals & Investigations, Chapter 17 34
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Forensic Science: Fundamentals & Investigations, Chapter 17 35
. . . . . . . . . . . . . Summary . . . .
o Ballistics is the study of bullets and firearms. o Firearms are divided into two groups—long guns
and hand guns. o Fired bullets show patterns of lands and grooves
that match the rifling pattern in the barrel. o A cartridge consists of primer powder, gunpowder,
a bullet and the casing material. o The caliber of a cartridge usually is a measure of
its diameter.
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Forensic Science: Fundamentals & Investigations, Chapter 17 36
. . . . . . . . . . . . . . . . . Summary
o Investigators also check for firing pin, breechblock, extractor, and ejector marks.
o Gunshot residue can help recreate a crime. o Using at least two reference points, an investigator
can recreate a bullets trajectory and determine where the shooter was located.
o Examining body wounds can determine where a bullet entered and exited the victim.