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Bullet Comparison and Identification (Casting (Procedure (choose…
Bullet Comparison and Identification
Identification
Physical Characteristics
Class characteristics - measurable features of a group by design factors
Individual Characteristics - made by random imperfections of tool
Subclass characteristics - group features from irregularity in manufacture (ie. mark in casting die)
Pattern Identification
determine if striated toolmarks of two objects correspond
somewhat subjective
Consecutive Matching Striae
quantitative method of describing observed pattern match
at least 2 groups of at least 3 consecutive matching striae in relatively same position; or 1 group of 6 consecutive matching striae in relatively same position
AFTE Theory of Identification
1985 - AFTE Criteria for Identification Committee formalized Theory of Identification as it Relates to Toolmarks
3 principles
enables opinions of common original when unique surface contours are in sufficient agreement
sufficient agreement determined by significant duplication of random toolmarks shown by corresponding pattern or combination of patterns on two surfaces
currently subjective - founded on science, based on examiner training and experience
Sufficient Agreement and Best Known Nonmatch
best known non-match is standard to be exceeded for degree of correspondence
examiner experience in recognizing corresponding matching striation patterns, corresponding impressed toolmark patterns, determining best known non-match, and comparing striated and impressed toolmarks
AFTE Range of Conclusions
Identification - sufficient agreement between two surface patterns exceeding those of best known non-match
Inconclusive
some agreement of individual and all class, but insufficient for identification
agreement of all class but not individual due to absence, insufficiency, or lack of reproduciblity
agreement of all class, disagreement of individual but not enough for exclusion
Elimination - sufficient disagreement of class or individual characteristics to exclude common source; more complicated with individual (need to know history and treatment of firearm - how it changed over time), so done
Unsuitable - bullet not in condition to be compared
Bullet Examination
Principles
unique characteristics mark bullet bearing surface when fired
individual characteristics reproducible and identifiable
barrel rifling has unique characteristics due to manufacturing, use, and wear
Situations
firearm, no evidence - obtain test bullets for alter comparison
firearm, with evidence - obtain test bullets, compare to recovered bullets to determine if common origin
evidence, no firearm - intercomparison to determine common origin
Microscopic Procedures
Firearm, no related evidence
determine if firearm reliably reproduces microscopic marks via test fire
assess quality of marks produced by firearm
retain physical samples of test fire bullets (compare with new evidence later, enter into digital image reference files)
Comparison Process - according to lab protocol; examinations should be done with bullets in phase
Firearm with related evidence
compare recovered evidence with test bullets recovered via lab SOP
conclude elimination, identification, or inconclusive
Evidence, no related firearm
intercomparison of recovered bullets
conclude elimination, identification, or inconclusive
Case Linkage
maintain files of fired evidence from unsolved cases
small percentage of criminal population commits large percentage of crimes - may find repeat offenders
submitted bullets compared to all previous bullets of similar caliber on file (unsolved cases, open cases, reference fired specimen files, open case ammunition files)
digital searches of files to find cases with higher probability of linking; then examiner confirms or excludes
now use Integrated Ballistics Identification System (IBIS)
Casting
may have to compare two bullets when one of the two is not accessible
causes
NIBIN hits made in remote lab that must be physically examined with comparison microscope
liaison with examiners from other jurisdictions on cases that may be related
informant tips off possible linkage between cases between jurisdictions
obstacles
chain of custody concerns
policies regarding the shipment of evidence between labs or jurisdictions
budget constraints regarding long distance travel between jurisdictions
solution: cast surface of bullet and ship to other examiner
cast must be compared to another cast, not the cased bullet itself
Procedure
choose recognized brand appropriate for toolmarks (can replicate microscopic detail)
mix components of casing material until mixture begins to harden
apply mixture to bullet and let harden completely
remove cast when hardened
Source: Savage K, Freed G. Firearm Examiner Training. National Forensic Science Technology Center. [accessed 2019 Jan 25].