Presentation on theme: "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."— Presentation transcript:
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
Forensic Science: Fundamentals & Investigations, Chapter 17 3 Introduction Ballistics is the study of bullets and firearms. 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
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 Cartridge and breech loading o Revolver, semi- automatic, and automatic handguns muzzle
Forensic Science: Fundamentals & Investigations, Chapter 17 5 Modern firearms are divided into 2 basic groups: Long Guns and Handguns o Long guns Rifles fire bullets Shotguns fire pellets (shot) or a single projectile (slug) Both require the use of 2 hands for accurate firing. o Handguns Pistols are fired with one hand Revolvers have a cylinder (that holds usually six cartridges) that turns as it is fired The Semiautomatic is loaded with up to 10 cartridges into a magazine (clip); fire only 1 bullet per pull of the trigger The Fully Automatic will fire repeatedly as long as the trigger is pressed
Forensic Science: Fundamentals & Investigations, Chapter 17 7 Firearms and Rifling o Grooves are indentations in the rifles barrel. The ridges (raised areas) that surround the grooves are called lands. o Grooves and lands in the barrel of a gun produce the twisting that adds accuracy. o The spiral pattern of lands and grooves in the barrel of a firearm is called rifling. o This leaves a pattern on the bullet that matches the barrel pattern to a specific firearm.
Forensic Science: Fundamentals & Investigations, Chapter 17 8 Bullets, Cartridges, and Calibers o Cartridgea 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. headstamp
Forensic Science: Fundamentals & Investigations, Chapter 17 9 How a Firearm Works 2. The primer powder sparks through the flash hole to the main propellant (gunpowder) supply. 1. Pull the trigger and the firing pin hits the base of the cartridge, igniting the primer powder mixture.
Forensic Science: Fundamentals & Investigations, Chapter How a Firearm Works 4. The bullet follows the lands and grooves pattern of the barrel and begins to spiral before it leaves the barrel. 3. The main gunpowder ignites, and the pressure of the explosion pushes the bullet from the casing into the barrel.
Forensic Science: Fundamentals & Investigations, Chapter Caliber of the Cartridge o Bullets (and their cartridges) are named by caliber and length. o Calibera measure of the diameter of the cartridgeby hundredths of an inch. (Europeans use the metric system-9 mm) o Common calibers include:.22,.25,.357,.38,.44, and.45 o A.45 caliber cartridge measures 45/100 of an inch in diameter (almost ½ an inch). o Caliber also refers to the diameter of the inside of a firearms barrel; therefore, the caliber of ammunition should match the firearm that shoots it. o If a bullet is removed from a wound or crime scene, its caliber can link it to the weapon used to fire it.
12 The Study of Bullets and Cartridge Casings 1. How is each fired bullet marked? As a gun is fired, the barrel marks each bullet with its own unique pattern of lands and grooves. 2. What is the procedure to match a spent bullet to the firearm that shot it? Investigators compare bullets and spent cartridges shot from the suspected firearm. To get a known bullet for comparison, they test-fire the weapon. 3. What makes up a test-firing, and why is it done? Investigators test-fire the weapon into a water tank or gel block. This captures the bullet without damaging it. Then, they can compare the markings on known bullets with those on the suspect bullets.
13 Marks on the Spent Cartridge Casings o Firing pin marks impressions made on the bottom of the cartridge by the firing pin as it strikes the bottom of the cartridge 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 The markings are unique to the firearm Can be matched if the spent cartridge casings are found
Marks (cont.) o Extractor marks-- minute scratches produced as the cartridge is placed in the firing chamber (by the extractor) o Ejector marks-- minute scratches produced as the cartridge is removed from the chamber (by the ejector) These marks are produced only in semiautomatic and fully automatic weapons In revolvers, cartridges are hand-fed into the revolving cylinder and have to be removed by hand as well Forensic Science: Fundamentals & Investigations, Chapter 17 14
Databases o A searchable collection of information stored in a computer system o Firearms databases can be searched to match crime-scene evidence to registered weapons o 2 important databases: NIBISNational Integrated Bullet Identification System (ballistic markings of firearms used in previous crimes) Drugfireand FBI database (focuses on cartridge casings) In 2000, these databases were merged to form NIBIN--National Integrated Ballistics Network 15
How is NIBIS used to help solve crimes? o Law enforcement agents can use this database to see if they can match a bullet found at a crime scene to a firearm that was used to commit a crime in the past. o If this connection is made, how could this information help a law enforcement agent? Forensic Science: Fundamentals & Investigations, Chapter 17 16
17 Gunshot Residues o Gunshot Residues (GSR) Particles of unburned powder and traces of smoke Traces are left on the hand, arm, face, hair, or clothing of the shooter and/or victim (evidence) o Chemical testing can detect residue even if removal (washing) is attempted o Distance from victim to shooter can be determined by examining the residue pattern on the victim The amount of GSR decreases as the distance between firearm and victim increases
Forensic Science: Fundamentals & Investigations, Chapter Trajectory o Two reference points are needed to define the trajectory o Investigators can figure the shooter discharged the firearm somewhere along that line
Forensic Science: Fundamentals & Investigations, Chapter 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
Forensic Science: Fundamentals & Investigations, Chapter Trajectory and Gravity o Bullets path is slightly curved o Gravity pulls it downward as the bullet moves forward Diagram is highly exaggerated
Forensic Science: Fundamentals & Investigations, Chapter 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?
Forensic Science: Fundamentals & Investigations, Chapter 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 Soor o AB = 732.3
Forensic Science: Fundamentals & Investigations, Chapter Triangulation o Using Pythagoreans theorem AB 2 = AC 2 + BC 2 o = BC 2 o BC 2 = – o BC 2 = – o BC = (square root) o BC = 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
24 Bullet Wounds – Questions to Discuss… Examining body wounds can determine where a bullet entered and exited the victim, which help determine where the shooter was located by identifying the bullets path (or trajectory). 1. Why do entrance wounds tend to be smaller than exit wounds? 2. If the bullet penetrates clothing, what can fibers embedded in the wound indicate? 3. Where on the victim is gunshot residue usually found? 4. If the gun is fired with the muzzle touching the victims skin, what telltale mark may show up? 5. Will larger or will smaller caliber bullets tend to lodge within the body rather than passing through? Why?
Answers 1. Generally, the size of the entry would will be smaller than the bullet because skin in somewhat elastic (it stretches when a bullet enters the body). Exit wounds are generally larger because as the bullet moves through the body, it may collect and carry body tissue and bone with it. 2. Fibers imbedded in wounds can indicate the direction of penetration. 3. Gunshot residue can be found only around entrance wounds. 4. If the gun is fired with the muzzle touching the victims skin, the hot gases released from the muzzle flash may burn the skin, leaving a telltale mark. 5. High-speed bullets are more likely to pass through a body than are low-velocity bullets. Therefore, small-caliber bullets, such as a.22 caliber, tend to lodge within the body, while larger-caliber bullets will pass through. Forensic Science: Fundamentals & Investigations, Chapter 17 25
Forensic Science: Fundamentals & Investigations, Chapter Summary Summary.... o Ballistics is the study of bullets and firearms. o Firearms are divided into two groupslong 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.
Forensic Science: Fundamentals & Investigations, Chapter Summary 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.