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
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Forensic Science: Fundamentals & Investigations, Chapter 17

2. 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.
Whether a weapon was fired recently.
If a firearm was used in previous crimes.
Forensic Science: Fundamentals & Investigations, Chapter 17

3. History of Gunpowder and Firearms
The Chinese invented gunpowder over a thousand years ago.
Muzzle-loading matchlocks used wicks to ignite the gunpowder.
The cartridge and breech loading followed.
Rifling provided greater accuracy.
Revolver, semi-automatic, and automatic handguns were developed.
Forensic Science: Fundamentals & Investigations, Chapter 17

4. Firearms and Rifling
Grooves and ridges (lands) in the barrel of a gun produce the twisting that adds accuracy.
This leaves a individualized pattern on the bullet.
Forensic Science: Fundamentals & Investigations, Chapter 17

5. 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.
Forensic Science: Fundamentals & Investigations, Chapter 17

6. 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.
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.
Forensic Science: Fundamentals & Investigations, Chapter 17

7. 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.
Why should the caliber of ammunition match the firearm that shoots it? If they do not match, what could go wrong?
Forensic Science: Fundamentals & Investigations, Chapter 17

8. The Study of Bullets and Cartridge Casings
Matching grooves (indentations)
Matching lands (elevations)
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?
Forensic Science: Fundamentals & Investigations, Chapter 17

9. 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 center of the spent cartridge.
Breechblock marks are produced as the cartridge casing slams backward and strikes the breechblock.
Other marks left on spent cartridge casings include minute scratch extractor and ejector nicks.
Forensic Science: Fundamentals & Investigations, Chapter 17

10. Gunshot Residues
Particles of unburned powder and traces of smoke are the residues of gunshots.
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.
Forensic Science: Fundamentals & Investigations, Chapter 17

11. Trajectory Two reference points are needed to define the trajectory.
Path of bullet
Horizon
Wind shield
Distance along path of
bullet to window, 23.9”
Distance along
horizon to
window, 23.5” y x
60 feet
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

12. 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.
Forensic Science: Fundamentals & Investigations, Chapter 17

13. Trajectory Determining the Location of the Shooter
Using the illustration on Slide 11 and adding that the shot came from a nearby building, these conclusions can be made:
Since the building is about 60 feet away, the shooter was about 11 feet above the bullet hole in the seat, which was 4 feet above the ground.
This height of about 15 feet off the ground puts the shooter on the second floor in that building.
Forensic Science: Fundamentals & Investigations, Chapter 17

14. Bullet Wounds
Why do entrance wounds tend to be smaller than exit wounds?
If the bullet penetrates clothing, what can fibers embedded in the wound indicate?
Where 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?
Forensic Science: Fundamentals & Investigations, Chapter 17

15. Summary
Fired bullets show patterns of lands and grooves that match the rifling in the barrel.
The caliber of a cartridge usually is a measure of its diameter.
Investigators also check for firing pin, breechblock, extractor, and ejector marks.
Gunshot residue can help recreate a crime.
It’s important to locate where the shooter was located.
Forensic Science: Fundamentals & Investigations, Chapter 17