chapter 17 notes

Transcription

chapter 17 notes

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
1
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
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
3
History of
Gunpowder and Firearms
o
o
o
4
Chinese invented
gunpowder over a
thousand years ago
Muzzle-loading
matchlocks used wicks
to ignite the gunpowder
Flintlock weapons used
a spark from a chip of
flint
Percussion firing weapons
o
o
o
5
Cartridge – holds bullet, primer powder,
gunpowder
Hammer hits primer powder which ignites the
gunpowder
Breech loading is faster than muzzle loading.
Long Guns and Handguns
o
Long guns
•
•
o
Handguns
•
•
o
6
o
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
Semi-automatic – 10 cartridges into a
magazine/clip. Fires one bullet per pull of trigger
Automatic – fires repeatedly as trigger is pulled
Firearms and Rifling
o
o
7
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
Bullets, Cartridges, and Calibers
o
o
8
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.
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
9
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
10
Caliber of the Cartridge
11
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?
The Study of Bullets
and Cartridge Casings
12
Matching bullets
o
o
o
13
Gun is “test-fired” into gel or water.
Doesn’t damage bullet
Markings are compared to suspect’s
weapons, bullets, casings.
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
•
14
•
Only in semi-auto and automatic weapons
Small scratches
15
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
16
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
Bullet Wounds
o
Can show what happened during the crime
•
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
GSR found near entrance wounds
Muzzle may burn skin if gun was close
o
Small caliber bullets tend to remain lodged in the body
o
o
17
Was the victim running away?
Trajectory
o
o
18
Two reference points are needed to define the
trajectory
Investigators can
figure the shooter
discharged the
firearm somewhere
along that line
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
19
Lasers can trace a straight-line path to
determine the position of the shooter
Trajectory and Gravity
o
o
o
o
Bullet’s path is
slightly curved
Gravity pulls it
downward as the
bullet moves forward
Wind can affect
trajectory
Bullet can hit other
objects and ricochet
Diagram is highly exaggerated
20
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?
21
Triangulation
o
o
o
o
22
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
=
So
or
Ac AC
AB = 732.3”
23.9" AB
=
23.5" 720"
Triangulation
23
o
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
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
Activity 17-1
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
o
24
Activity 17-1
o
25
Use Law of Tangents to find the height of the
shooter above the bullet
. . . . . . . . . . . . . Summary . . . .
o
o
o
o
o
26
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.
. . . . . . . . . . . . . . . . . Summary
o
o
o
o
27
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.

chapter 17 notes

Transcription

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