fired casings usually more helpful than fired bullets; more found, microscopic marks, in better shape than bullet

fired: firing pin impression, breech face marks, and chamber marks

mechanism marks can have unique microscopic characteristics

the individual characteristics may be uniquely identified with particular firearm

optical systems and peripheral equipment for comparison microscopy

Lab Policies - chain of custody, lab safety, trace evidence, evidence marking and packaging

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

other: firing range, ammunition, containers, scribes, stereomicroscope, microscope lighting, video monitor, imaging equipment

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

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

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 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 (CMS) used for qualitative pattern matching

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

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

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