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Cartridge and Shotshell Examination (Identification Process (Possible…
Cartridge and Shotshell Examination
Identification
fired casings usually more helpful than fired bullets; more found, microscopic marks, in better shape than bullet
Evidentiary Value
unique striations and impressions
fired: firing pin impression, breech face marks, and chamber marks
unfired: extractor and ejector marks, other mechanism marks
Individual Characteristics
mechanism marks can have unique microscopic characteristics
the individual characteristics may be uniquely identified with particular firearm
Developments
optical systems and peripheral equipment for comparison microscopy
digital imaging and management
improved criteria for identification
improved hardware and software databases
digital ammunition reference material
Prerequisites for Examination
Lab Policies - chain of custody, lab safety, trace evidence, evidence marking and packaging
Documentation per SOP; worksheet
Equipment - stereo and comparison microscope clibration
Class characteristics via SOP
Microscopic Marks - used for comparison and may be used for identification
Test standards - known standards obtained, marked, and preserved; collected in database; goal to determine cartridge types and manufacturer
Equipment
comparison microscope main tool for comparison
other: firing range, ammunition, containers, scribes, stereomicroscope, microscope lighting, video monitor, imaging equipment
Identification Process
tool - harder of two objects in contact, makes mark on softer object
cycle of fire: feeding, chambering, locking, firing, obturation, unlocking, extraction, ejection, cocking
striations - contour variations on surface from two objects moving against each other under pressure
impressed tool marks - occur when tool place against another object and force applied
Possible Toolmarks
Firing Pin Impressions - indentation where firing pin contacts cartridge
Firing pin drag marks - striations from firing pin contacting cartridge during extraction and ejection
Breech face marks - negative impression of breech face on head of case
Primer shearing marks - striations from margins of firing pin hole scraping primer metal during breech locking
Chamber marks - microscopic marks on case from chamber wall due to chambering, expansion during firing, or extraction
Extractor marks - striations on cartridge from extractor; usually on or just ahead of rim
Ejector marks - toolmarks on head of case from contact with ejector
Anvil marks - microscopic marks on forward face of rim for rimfire cartridges when forced against breech end of chamber by firing pin
Ejection port marks - striations from contact between ejection port and cartridge case
magazine marks - striations on periphery of case as it moves from lips of magazine to chamber during feeding
Physical Characteristics
class characteristics - features restricted to a group; from design
individual characteristics - marks made by random imperfections on tool; unique to one thing
subclass characteristics - surface features that are more restrictive than class; produced incidental to manufacture, relate to a smaller group source, and identifiable within a time frame of manufacturing
Pattern Identification
Pattern Matching - process of seeing if details of pattern correspond between multiple objects
start with class characteristics for exclusion; then do individual
in individual, look for repetitive marks occurring in both objects in same location and position
more qualitative than quantitative; relies on experience, training, and education of examiner; somewhat subjective
Consecutive Matching Striae
consecutive matching striae (CMS) used for qualitative pattern matching
written criteria established to define CMS
AFTE Theory of Identification
formalized 1985 by Criteria for Identification Committee
principle 1: enables opinions of common origin to be made when unique toolmarks of two surfaces agree sufficiently
principle 2: sufficient agreement based on significant duplication of random toolmarks via correspondence of pattern of surface contours
principle 3: interpretation subjective in nature; founded on science and based on examiner training and experience
Sufficient Agreement and Best Known Match
recognize corresponding patterns of striations and impressed toolmarks
determine bets known non-match in personal experience of examiner
training and experience comparing striated and impressed toolmarks
AFTE Range of Conclusions
Identification - individual characteristics sufficient to determine toolmarks have common origin
Inconclusive - agreement of class characteristics and some or no individual characteristic agreement such that a common origin cannot be either eliminated or identified
Elimination - significant disagreement of class or individual characteristics such that a common origin may be excluded; for individual characteristics, it must be shown that the firearm cannot have undergone sufficient use or abuse to account for the differences
Unsuitable - the evidence is not in a condition to be able to determine a common origin
Fired Case Examination
Microscopic Procedure
review lab protocols as needed
complete administrative requirements (chain of custody, QA, etc.)
follow lab safety protocols
determine the presence of trace evidence and follow lab protocol for collection
determine if latent fingerprint examinations should be preformed prior to toolmark examination
determine if other (not requested) examinations should be performed
Microscopic Comparison
ensure microscope meets maintenance and calibration standards
select appropriate magnification; lock lenses in place
select appropriate magnification for ocular lenses
select and orient light source
Firearms without Related Evidence
mount test case on right stage with base pointed up
orient light to illuminate marked area of base obliquely
examine base and primer area at low magnification for area of best individual characteristics
mount another test case on left stage in same orientation as case on right stage
obliquely light second case
move stages to align corresponding marks; draw conclusions
examine and compare firing pin impressions
draw conclusions as needed based on microscopic detail
examine extractor marks, ejector marks, chamber marks, magazine lip marks, anvil marks, ejection port marks, etc.
draw conclusions as needed based on microscopic detail
Firearms with Related Evidence
determine if case has marks of value for comparison
determine if test cases can be identified with each other
mount best test case on right stage with base pointed up; light obliquely
examine base and primer area at low magnification for area of best individual characteristics
mount evidence case on left stage, orient and light same as right stage
confirm cases have same class characteristics
move stages to align microscopic marks; document area of best agreement; draw conclusions as needed
if evidence case is damaged, use other areas of evidence to test items for comparison
examine firing pin impressions and draw conclusions as needed
examine extractor, ejector, chamber, magazine lip, anvil, ejection port, and all other marks; draw conclusions as needed
Evidence without Related Firearm
determine if case has microscopic marks of value for comparison
mount best evidence case on right stage, base pointed upward, and orient light obliquely
examine base and primer area for area of best individual characteristics
mount another evidence case on left stage in same orientation and lighting as right stage
confirm cases have same class characteristics
move stages to align microscopic marks; draw conclusions as needed; document area of best agreement
if cases damaged, use other areas of evidence and test items for comparison
examine firing pin impressions; draw conclusions as needed
examine extractor, ejector, chamber, magazine lip, anvil, ejection port, and other mechanism marks; draw conclusions as needed
Examination Challenges
Cartridges
inappropriate interchangeability - may lead to incorrect exclusion
Cartridge Cases
reloading toolmarks - marks from holding, resizing, and crimping
manufacture and assembly factors - marks on primers, bunter marks (made by die that makes headstamp)
Firearms
use of adapters - inserted into barrel to use alternative cartridges
substitution of parts - replacing pistol components, revolver cylinder replacement
modifications - refiling of parts
manufacturer-related factors - subclass characteristics, ultrasmooth machining techniques (absence of marks)
Examination of Unfired Ammunition
use extractor, ejector, and magazine marks for identification
can determine if cartridge was loaded into and extracted from a particular firearm
can determine if cartridge was loaded into and removed from a given magazine associated with a firearm
cartridge may have been cycled through without fire to remove cartridge as normal part of unloading, clear misfire, clear misfeed, unconscious "courage marks", etc.
can determine type of firearm it was intended for or compatible with even if identification is not possible
worksheets
similar to those of fired cases
write "not applicable" for chamber marks and breech face marks
write "none observed" for firing pin impression
Case Linkage
labs keep files on fired evidence from unsolved cases; compare to new cases to determine if linked
evidence files: unsolved, open, reference fired specimen, open case ammunition
files now more limited to particular years and cases for storage capacity reasons
Advanced Case Linkage and Fired Cartridge Case Comparisons
computers and optical hardware; software for special algorithms, pattern recognition, and digital imaging
used to screen firearms evidence for linkages
Integrated Ballistics Identification Systems (IBIS) - current predominant system; developed by Forensic Technology Incorporated (FTI)
after computer link established, evidence is compared by examiner
Source: Savage K, Freed G. Firearm Examiner Training. National Forensic Science Technology Center. [accessed 2019 Jan 25].