Module 5B - Education Service Center Region 13

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Module 5B - Education Service Center Region 13

Transcript Of Module 5B - Education Service Center Region 13

Module 5B
Bullets, Cartridge Cases, & NIBIN
Forensic Science Teacher Professional Development

Unit 2 The Class and Individual Characteristics of Bullets and Cartridge Cases
A cartridge consists of a primer, cartridge case, powder or propellant, and bullet. The following identify the process of firearm discharge.
1. When a firearm is fired, the firing pin strikes the primer of the cartridge.
2. This action ignites the propellant and produces an extreme amount of high-pressure gas within the cartridge.
3. This pressure causes the cartridge to swell and occupy the diameter of the firearm’s chamber.
4. This obturation effectively seals the resulting gasses behind the bullet and in front of the cartridge case.
5. As the gasses expand at an incredibly rapid rate, the cartridge case slams back into the breech face of the firearm, and the bullet is rapidly projected down the barrel of the firearm and out of its muzzle.

Unit 2 The Class and Individual Characteristics of Bullets and Cartridge Cases
 The fired cartridge case will bear striation and impression marks as a result of contact with the breech face and the firing pin.
 The fired bullet will bear striations on land engraved areas (LEAs) as a result of traveling down the barrel of the firearm, as shown in Figure 6.
 Fired bullets and cartridge cases can be linked to the firearm from which they were fired by matching individual characteristics found in these striated and impressed marks.

Unit 2 The Class and Individual Characteristics of Bullets and Cartridge Cases
Class characteristics of fired bullets  The number of lands and grooves in a firearm barrel  The widths of the lands and grooves  The direction in which the lands and grooves spiral (the
“twist”), either right or left  The caliber, or the distance between two opposing lands
in hundredths of an inch (US) or in millimeters

Unit 2 The Class and Individual Characteristics of Bullets and Cartridge Cases
Figure 6 Rifling in this barrel has eight lands and grooves that twist to the left (top photo).
This discharged bullet exhibits eight land and groove impressions with a left twist (bottom photo).
Image: COPYRIGHT A. L. Davis/VA DFS
Visit the following website to find out more on how a bullet can be traced to a particular gun. http://www.scientificamerican.com/article.cfm?id=how-can-a-bullet-be-trace

Unit 2 The Class and Individual Characteristics of Bullets and Cartridge Cases
Computer Databases  The FBI maintains a central computerized firearm file
called the General Rifling Characteristic file (GRC).  Firearm examiners can search the file with the caliber,
number of lands and grooves, and the widths and direction of twist. This may help to determine the make and model of the firearm that could have fired the crime scene bullet.

Unit 2 The Class and Individual Characteristics of Bullets and Cartridge Cases
Individual Characteristics  Striations found on bullets have individual characteristics
that can be identified back to a particular firearm because of microscopic imperfections within the barrels of firearms.  Firearm barrels are machined to have lands and grooves in order to improve their accuracy by imparting a stabilizing spin to the bullet.  Machining methods that create these lands and grooves, such as cutting, button swaging, and hammer forging, leave different microscopic imperfections within barrels.

Unit 2 The Class and Individual Characteristics of Bullets and Cartridge Cases
Individual Characteristics, continued  Previous research has shown that even consecutively
manufactured barrels will have such random imperfections that bullets fired through these barrels can be successfully linked with the correct barrel.  Firearms examiners are tasked with locating the microscopic individual characteristics found on fired bullets and cartridge cases. They then determine if the bullets or cartridge cases came from a particular firearm.

Unit 2 The Class and Individual Characteristics of Bullets and Cartridge Cases
Individual Characteristics, continued  Imperfections in the lands
during manufacture or ones caused by wear, nicks, scratches etc., will be mirrored on the surface of the fired bullets.  These rifling imperfections are unique to each barrel and form the basis for a projectile-to-barrel identification.
Figure 7 Striations on the surface of a fired bullet
BulletsCharacteristicsLandsFirearmClass