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

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

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

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

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.

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

Firearms Automatic and Semi-automatic

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.

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.

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

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?

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.

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?

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.

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.

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

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.

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.

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.

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

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

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

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.

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.

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.

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.

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.

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.

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.

. . . . . . . . . . . . . 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.

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