Questions and Answers
Class characteristics are measurable features that indicate a restricted group _____
source
What is the purpose of the extractor?
To withdraw the cartridge or cartridge case from the chamber.
Which of the following firing pin shapes is not very common?
Firing pin impressions can only be found on rimfire cartridges.
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Match the following rifling types with their characteristics:
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What are class characteristics of breechface marks?
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What is one method used to manufacture firearm barrels?
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Each barrel has class characteristics such as caliber, direction of twist, and type of _____
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What does the FBI's GRC file help to determine?
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Ejectors consistently leave marks on cartridges.
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Study Notes
Class Characteristics
- Class characteristics are measurable features of a specimen that indicate a restricted group source, determined before manufacturing.
- These characteristics can eliminate potential sources of evidence during forensic investigations.
Cartridge Cases and Breech Face
- Cartridge cases feature breech face marks including firing pin impressions and extractor relationships; marks can be impressed or striated.
- The breech face supports the cartridge head during firing, transferring unique manufacturing marks onto the cartridge case.
Breechface Marks
- Types of breaching marks include parallel, circular, arched, granular, cross-hatched, and smooth; the mark's direction can vary.
- Circular marks are made by end mills and may not always be unique; inconsistencies should be closely examined.
Firing Pins
- Firing pin shapes influence corresponding markings on cartridges, which are created during the machining process.
- Firing pins are crucial in initiating ignition by striking the cartridge's primer or rim.
Firing Pin Impressions
- Impressions are found in rimfire cartridges and centerfire cartridges, showing the impact of the firing pin on ignition.
Firing Pin Shapes
- Common shapes for centerfire cartridges include hemispherical, circular, elliptical, and rectangular; rimfire cartridges mainly feature rectangular, hemispherical, and circular shapes.
Extractors and Ejectors
- Extractors withdraw cartridges from the chamber, leaving striated marks on the rim, grooves, or during manual operations.
- Ejectors kick cartridges from the firearm, producing marks as they make contact with the cartridge case during ejection.
Class Characteristics of Barrels
- Barrels have distinct class characteristics including caliber, direction of twist, type of rifling, number of lands and grooves, and width of the lands and grooves.
- Similar calibers may overlap in characteristics due to minor dimensional differences.
Measuring Class Characteristics
- Important measurements include caliber, rifling type, twist direction, and impressions of lands and grooves.
Barrel Manufacturing
- Grooves in barrels, known as rifling, are shaped using techniques such as broach cutting, button rifling, and hammer forging, resulting in class characteristics unique to each barrel.
- Conventional rifling features sharp-edged grooves; polygonal rifling has rounded profiles.
FBI's GRC File
- The GRC file assists in determining the make/type of firearm that fired a bullet based on general rifling characteristics, containing data from numerous firearms.
Flow Back Phenomenon
- Flow back occurs when the primer melts from heat during firing, causing elements to flow through the aperture; some firearms may be more prone to this issue.
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Description
Explore the fundamental class characteristics relevant to forensic analysis in week 3 of the FA ID course. This set of flashcards covers crucial definitions and concepts such as breech face marks and cartridge cases, essential for understanding evidence in firearm identification.
