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Module 9: Cartridge and Shotshell Examination (Possible Toolmarks (firing…
Module 9: Cartridge and Shotshell Examination
Identification
fired cartridge cases are frequently more useful than fired bullets when linking shooting incidents
they are usually found in greater quantity at crime scenes
bear more microscopic marks of value
microscopic marks on bullets are subject to
mutilation
deformation
fragmentation
dates to 1925 when a comparison microscope was modified to compare unique marks produced by firearms on the surface of ammunition
based on reproducible striations and or/impressions produced during normal functioning of a firearm
cartridge cases and shotshell cases as having been fired in a particular firearm
firing pin impressions
breech face marks
chamber marks
fire and unfired cartridge cases and shotshell cases as having been cycled through the action of a particular firearm
extractor and ejector marks
other mechanism marks
since 1925, significant advances in identification have been made
optical systems and peripheral equipment for comparison microscopy
digital imaging and management
more formal and structured definitions of the criteria for identification
advances in computer hardware platforms and database software to link serial shooting
digitalized ammunition reference resources available on the Internet and on storage media
Identification Process
firearms identification is simply a firearms-related specialized subset of the broader field of toolmark identification
cycle of fire includes
feeding of a cartridge manually or from a magazine
chambering of a cartridge
locking of the breech or bolt mechanism
firing by releasing of the firing pin
obturation, specifically, sealing of powder gases forward of the cartridge cases or shotshelll case by expansion of the cartridge case to fully fill the chamber, as well as by expansion of projectiles or wadding material
unlocking of the breech or bolt mechanism
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pattern matching
the process of determining whether or not the details of striated marks or impressions on two objects correspond
match land and groove impressions of evidence and standard
Prerequisites for Examination
lab policies are to be followed
chain of custody
lab safety
evidence marking and packaging
trace evidence
basic descriptive information should be documented prior to any examination or comparison
stereo and comparison microscopes should be calibrated and maintained prior to examination
determination of class characteristics is performed according to lab protocol
test standards are obtained and appropriately marked and preserved
Equipment
comparison microscope is the most basic tool for examination of evidence
other equipment that may expedite the process
range facilities
variety of ammunition
appropriate containers for fired test standards
metal or electric scribes and permanent markers
stereomicroscop
microscope lighting options
imaging equipment
Possible Toolmarks
firing pin impressions
firing pin drag marks
breech face marks
primer shearing marks
chamber marks
extractor marks
ejector marks
ejection port marks
magazine marks
Identification Results
inconclusive
some agreement of individual characteristics and all discernible class characteristics, but insufficient for identification
elimination
significant disagreement of discernible class characteristics and/or individual characteristics
unsuitable
unsuitable for comparison
appropriate for fired and mutilated cartridge cases and shotshell cases that do not bear microscopic marks of value for comparison purposes
Examination Challenges
cartridges
inappropriate interchangeability
cartridge cases
reloading toolmarks
manufacture/assembly factors
firearms
use of adapters
substitution of parts
modifications
manufacturer-related factors