1. Ballistics By the end of this chapter you will be able to:
Explain the differences between a handgun, a rifle, and a shotgun
Describe rifling on a gun barrel and how it affects the flight of the projectile
Explain barrel size and caliber
All Rights Reserved South-Western / Cengage Learning © 2012, 2009

2. Ballistics By the end of this chapter you will be able to:
Describe how bullets are test fired and matched
Discuss the role of ballistics recovery and examination at a crime scene

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

4. History of Gunpowder and Firearms
Chinese invented gunpowder over a thousand years ago
Muzzle-loading matchlocks used wicks to ignite the gunpowder
Cartridge and breech loading
Revolver, semi-automatic, and automatic handguns

5. Firearms Long guns Rifles - fire bullets
Shotguns fire pellets (shot) or a single projectile (slug)
Unlike rifled firearms, a shotgun has a smooth barrel. The diameter of the shotgun barrel is expressed by the term gauge. The higher the gauge number, the smaller the barrel’s diameter.

6. Firearms Handguns Pistols are fired with one hand
Revolvers have a cylinder that
holds usually six cartridges

7. Firearms
Automatic and Semi-automatic

8. Gun Barrel Markings
The inner surface of the barrel of a gun leaves its markings on a bullet passing through it.
These markings are unique to each gun.
The gun barrel is produced from a solid bar of steel that has been hollowed out by drilling.
The microscopic drill marks left on the barrel’s inner surface are randomly irregular and serve to impart a uniqueness to each barrel.

9. Gun Barrel Markings
The manufacture of a barrel also requires impressing its inner surface with spiral grooves, a step known as rifling.
The surfaces of the original bore remaining between the grooves are called lands.
The grooves serve to guide a fired bullet through the barrel, imparting a rapid spin to insure accuracy.

10. The diameter of the gun barrel, measured between opposite lands, is known as the caliber.
Groove

11. Caliber of the Cartridge
Caliber—a measure of the diameter of the cartridge
In hundredths of an inch
Common calibers include .22, .25, .357, .38, .44, and .45
Why should the caliber of ammunition match the firearm that shoots it? If they do not match, what could go wrong?

12. Bullets, Cartridges, and Calibers
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.

13. The Study of Bullets and Cartridge Casings
How is each fired bullet marked?
What is the procedure to match a spent bullet to the firearm that shot it?
What makes up a test-firing, and why is it done?

14. Bullet Examination
NO two rifled barrels, even those manufactured in succession, will have identical striation markings.
The number of lands and grooves and their direction of twist are obvious points of comparison during the initial stages of an examination between an evidence bullet and a test-fired bullet.
Any differences in these class characteristics immediately serve to eliminate the possibility that both bullets traveled through the same barrel.

15. Comparison Microscope
The comparison microscope serves as the single most important tool to a firearms examiner.
Two bullets can be observed and compared simultaneously within the same field of view.
Not only must the lands and grooves of the test and evidence bullet have identical widths, but the longitudinal striations on each must coincide.

16. Marks on the Spent Cartridge Casings
Firing pin marks
appear on the rim or center of the spent cartridge
can be used to match a cartridge to a firearm
Breechblock marks
produced when the cartridge casing slams backward and strikes the breechblock

17. Marks on the Spent Cartridge Casings
Extractor marks
mechanism in a firearm by which a cartridge of a fired case is withdrawn from the firing chamber.
Ejector marks
mechanism in a firearm that throws the cartridge or fired case from the firearm.
Magazine or clip
which is the mechanism that in a firearm holds the bullets.

18. Computerized Imaging
The advent of computerized imaging technology has made possible the storage of bullet and cartridge surface characteristics in a manner analogous to automated fingerprint files.

19. Computerized Imaging
The National Integrated Ballistics Information Network, NIBIN, produces database files from bullets and cartridge casings retrieved from crime scenes or test fires from retrieved firearms, often linking a specific weapon to multiple crimes.

20. How a Firearm Works
The firing pin hits the base of the cartridge, igniting the primer powder
The primer powder sparks through the flash hole to the main propellant supply

21. How a Firearm Works
The pressure of the explosion pushes the bullet from the casing into the barrel
The bullet follows the lands and grooves spiraling out of the barrel

22. Gunshot Residues 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
Chemical testing (Greiss Test) can detect residue even if removal is attempted
Distance from victim to shooter can be determined by examining the residue pattern on the victim

23. Primer Residue on Hands
The firing of a weapon not only propels residues toward the target, but gunpowder and primer residues are also blown back toward the shooter.
As a result, traces of these residues are often deposited on the firing hand of the shooter, and their detection can provide valuable information as to whether or not an individual has recently fired a weapon.

24. Primer Residue on Hands
Examiners measure the amount of barium and antimony on the relevant portion of the suspect’s hands, such as the thumb web, the back of the hand, and the palm.
They may also characterize the morphology of particles containing these elements to determine whether or not a person has fired, handled a weapon, or was near a discharged firearm.

25. Serial Numbers
Increasingly, the criminalist is requested to restore a serial number when it has been removed or obliterated by grinding, rifling, or punching.
Restoration of serial numbers is possible through chemical etching because the metal crystals in the stamped zone are placed under a permanent strain that extends a short distance beneath the original numbers.

26. Firearm Evidence Collection
Firearms are collected by holding the weapon by the edge of the trigger guard or by the checkered portions of the grip.
Before the weapon is sent to the laboratory, all precautions must be taken to prevent accidental discharge of a loaded weapon.
In most cases, it will be necessary to unload the weapon.

27. Firearm Evidence Collection
When a revolver is recovered, the chambers, their positions, and corresponding cartridges must be recorded.
Firearm evidence must be marked for identification (usually a tag on the trigger guard) and a chain of custody must be established.

28. Firearm Evidence Collection
Bullets recovered at the crime scene are scribed with the investigator’s initials, either on the base or the nose of the bullet.
The obliteration of striation markings that may be present on the bullet must be scrupulously avoided.
The investigator must protect the bullet by wrapping it in tissue paper before placing it in a pillbox or an envelope for shipment to the crime laboratory.

29. Firearm Evidence Collection
Fired casings must be identified by the investigator’s initials placed near the outside or inside mouth of the shell.
Discharged shotgun shells are initialed on the paper or plastic tube remaining on the shell or on the metal nearest the mouth of the shell.

30. Summary
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.

31. Summary
Investigators also check for firing pin, breechblock, extractor, and ejector marks.
Gunshot residue can help recreate a crime.