forensic science: fundamentals & investigations, chapter 17 1 chapter 17 ballistics by the end...

Forensic Science: Fundamentals & Investigations, Chapter 17 1

Chapter 17 Ballistics By the end of this chapter you will be able to:

describe rifling on a gun barrel and explain how it marks a bullet

explain barrel size and caliber describe how bullets are test fired and matched discuss the role of ballistics recovery and

examination at a crime scene determine the position of the shooter based on bullet

trajectory

All Rights Reserved South-Western / Cengage Learning © 2009


Introduction Introduction

Ballistics – the study of bullets and firearms.

Firearms – weapon capable of firing a projectile using a confined explosive as a propellant.

Ballistic evidence helps explain: What type of firearm was used. The caliber of the bullet. The number of bullets fired. Where the shooter was. Whether a weapon was fired recently. If a firearm was used in previous crimes.


History of History of Gunpowder and Firearms Gunpowder and Firearms

The Chinese invented gunpowder (KNO3, charcoal, & sulfur) over a thousand years ago.

Muzzle-loading matchlocks, the earliest firearms, used wicks to ignite the gunpowder.

The cartridge (a case that holds a bullet, primer powder, and gunpowder) followed.

Cartridges were loaded into the breech – the end of the barrel attached to the firing mechanism.


Long Guns and Handguns

Long guns, i.e. rifles and shotguns, require two hands for accuracy

Rifles fire bullets; shotguns fire small, round pellets or a projectile called a slug.

Handguns fired with one hand are called pistols Revolvers are pistols with a revolving cylinder that

holds 6 cartridges Semiautomatics hold up to 10 cartridges in a

magazine (clip). Semiautomatic fires one bullet per pull of the trigger. Fully automatic fires repeatedly as long as the trigger

is pressed.

Assignment

Working in pairs or groups of three, research 5 different types of ammunition (not different calibers), describe each in terms of appearance and usage.



Vocabulary

Grooves – depressions Lands – ridges Both are found on the inside of a firearm’s

barrel and are created when the firearm is manufactured

Rifling – the spiral pattern of lands and grooves in the barrel of a firearm


Firearms and Rifling Firearms and Rifling

Grooves and lands in the barrel of a gun produce the twisting that adds accuracy.

This leaves a individualized pattern on the bullet. It is impossible to produce two identically rifled

barrels


Bullets, Cartridges, and Calibers Bullets, Cartridges, and Calibers

Bullets and cartridges are packaged together.

The bullet, usually of metal, is out front with the cartridge, holding the primer and propellant powders, behind.


Anatomy of a Cartridge

Bullet – composed of lead, copper, or combination of other metals. Can be metal-jacketed, hollow-point, plastic coated

Primer powder – initiates the contained explosion that pushes the bullet down the barrel

Anvil and flash hole – provide the mechanism of delivering the explosive charge from the primer powder to the gunpowder.

Headstamp – on bottom of cartridge identifies the caliber and manufacturer.


How a Firearm Works

1. Pulling the trigger causes the firing pin to hit the base of the cartridge, igniting the primer powder.

2. The primer powder sparks through the flash hole to the main gunpowder supply.

3. The main gunpowder supply ignites and 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.


Caliber of the Cartridge

Caliber is a measure the diameter of the cartridge.

These usually are hundredths of an inch. Common calibers include .22, .25, .357, .38,

.44, and .45. Europe uses metric scale i.e. 9mm

Caliber narrows down the type of gun used


The Study of Bullets and The Study of Bullets and Cartridge Casings Cartridge Casings

Investigators compare bullets and spent cartridges from a crime scene to those from the suspected firearm.

Weapon is test-fired into water or a gel block so bullet is captured without damage.

Matching grooves (indentations)

Matching lands (elevations)


Marks on the Spent Cartridge Casings

Firing pin marks on a spent cartridge can be used to match it to a firearm.

The fire pin marks can appear on the rim or on the bottom of the spent cartridge.

Breechblock marks are produced as the cartridge casing slams backward & strikes the breechblock.

Other marks on spent cartridge casings are extractor and ejector nicks on semi and fully automatic weapons caused by loading and ejection of the cartridges

Firing Pin Match

Activity 17-2 in text. Pg 512-513 Copy table on page 513 and fill it in looking

at the casings from three suspects Then make a list of all of the recovered

bullets and the suspect it matches (A=2, B=1, etc)

Answer Analysis questions and Further Study.



Gunshot Residues (GSR) Gunshot Residues (GSR)

GSR - Particles of unburned powder and traces of smoke after gun is fired.

They can leave a trace on the hand, arm, face, hair, or clothing of the shooter.

They can also leave a trace on the victim. Chemical testing often can detect residue even if

removal is attempted. The distance from the victim to the shooter can be

determined by examination of the residue pattern on the victim.


Databases

Firearms databases can be used to match crime scene evidence to registered weapons

National Integrated Bullet Identification System (NIBIS) records ballistics markings of firearms used in previous crimes

Drugfire – FBI database that focuses on cartridge casings

In 2000, they merged to form the National Integrated Ballistics Network (NIBIN)

Database Opinion Paragraph

Write a paragraph (at least 7 good, thoughtful sentences) about whether law enforcement should maintain a database of all firearms that are manufactured or sold in the U.S.



Calculating Trajectory Calculating Trajectory

Trajectory – the path of the propelled bullet Helps determine where the shooter was located. If trajectory angle is downward, the shooter was

positioned above the target.Two reference points along the flight path are

needed to define the trajectory. Investigators can figure the shooter discharged

the firearm somewhere along that line.


Trajectory Trajectory

Reference points can be bullet holes in objects or victims.

An entry point and exit point on a victim can be used.

Gunshot residue or spent cartridge casings can be less specific reference points.

Investigators can use lasers to trace a straight-line path to help determine the position of the shooter.


TrajectoryTrajectory

Can be difficult to determine Bullet can ricochet, become damaged, or

does not provide a direct path for measurement.

Targeting needs to be adjusted for very distant objects due to gravity.


Trajectory Trajectory

Distance to the window = Distance to the shooter (x)

Distance along horizon to window Distance to the building

With the distance to the building and the distance to the shooter along the trajectory of the bullet, the Pythagorean theorem can be used to determine the height of the shooter above the horizon (not the ground)

Path of bullet

Horizon

Wind shield

Distance along path of bullet to window, 23.9”

Distance along horizon to window, 23.5”

yx

60 feet


Bullet Wounds Bullet Wounds

Why do entrance wounds tend to be smaller than exit wounds?

If the bullet penetrates clothing, fibers may be embedded in the direction of penetration

GSR usually only found near entrance wounds. If gun is fired with the muzzle touching victim’s

skin, the muzzle flash may burn the skin leaving a mark


Bullet wounds

Bullets may not travel smoothly through body; tumbling effect due to bones, organs and other tissues encountered

High speed bullet are more likely to pass through

Small caliber bullets tend to lodge within the body (less mass, not necessarily lower speed)

forensic science: fundamentals & investigations, chapter 17 1 chapter 17 ballistics by the end...

Documents