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Characterization and Evaluation of Fired Projectiles
retrieved March,…
Characterization and Evaluation of Fired Projectiles
retrieved March, 2020
https://firearms-examiner.training.nij.gov/
Module 10
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Caliber/Diameter
measured diameter of fired bullets is taken from one groove impression (a high point on a fired bullet) to a groove impression on the opposite side of the bearing surface.
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Composition
- Solid Bullets
- Bullet Jackets
- Bullet Cores
- Bullet Coatings
- Sabots
- Full metal jacket
- Semijacketed
- Total metal jacket
Properties
- Magnetic - test if bullet is ferrous
- Length - fired bullets slightly longer than unfired ones
specialized color-coded bullets:
- Armor-piercing bullets
- Exploding bullets
- Frangible bullets
- Incendiary bullets
- Tracer bullets
Base Construction:
- Solid base bullet
- Open base
- Jacketed solid base
- Gas check bullet
Nose Construction:
- provide for exposure
- cavity in the nose to facilitate expansion
- Solid nose constructed of a single metal or alloy
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Equipment
Comparison Microscope
- air gap method can be used to measure land and groove impressions
- uses two stages
Stereomicroscope
- Place a fired evidence bullet and the micrometer under the microscope.
- Open the jaws of the micrometer to match the reference points on the fired bullet.
Reticules
Electronic:
- steps for measuring land and groove impressions using electronic reticules are the same as those for glass reticules
Glass:
- fixed mount that can be rotated.
- both of the eyepieces of the microscope must be adjustable to use this technique
Digital Equipment
MP 6 Optical Projector
- bullet image is projected onto a hooded screen that displays a measuring grid
- land or groove impression is displayed in an LED readout in thousandths of an inch
FBI GRC
File Standards
collection of well-documented, known GRC data that can be used as a standard for comparison of class characteristics of no-gun submission evidence bullets
Damaged Bullets
- In some cases, bullets bear no microscopic marks of value for comparison purposes because they are fragmented, distorted, deformed, or otherwise damaged.
Microscopic Marks
- must be evaluated for their potential
- present and sufficient
- present and insufficient
- not present
Rifling Impressions
- Polygonal rifling prohibits the examiner from measuring the width of land and groove impressions because the lands and grooves have a rounded profile instead of a well-defined rectangular profile
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