FORS-2107EL Lecture Notes - Lecture 4: Comparison Microscope, Trace Evidence, Firing Pin
27 Jun 2018
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Forensic Science - Day 4 2018.01.18.
-Descriptor factors is what we look for in evidence, a set of little pieces of information
in order to be able to identify or compare evidence
Action Marks
-These are individual factors that can help to individually distinguish the source of the
bullet (which gun)
-Impression Action Marks occur when a cartridge case is fired in a firearm, and the two
most common marks are firing pin impressions and breech marks, and sometimes
ejector marks
-Note that ejector marks can also be in the form of striated action marks or impression
action marks
-Striated action marks occur on cartridge cases that have passed through the action of
an auto loading or repeating firearm, they are “Scraping” essentially and are
produced by coming in contact with:
-Chamber, shear, firing pin drag marks, extractor marks, and ejector marks
-Ejector marks can be in the form of impression or striated action marks
Firing pin impressions
-These marks are created through the striking of the primer (centre for centre fire, rim
for rimfire) by the firing pin
-These can be unique characteristics if the tip of the firing pin has imperfections as
they will be impressed into the metal of the cartridge
Individual Characteristics of Firearms Evidence
-Individual characteristics are “marks produced by the random imperfections or
irregularities of tool surfaces. These random (microscopic) imperfections or
irregularities are produced incidental to manufacture and or caused by use, corrosion,
or damage. They are unique to that tool and distinguish it from all other tools” (AFTE
Glossary)
-This states that there are individual characteristics that will be found on tool surfaces,
that don’t have to do with the manufacturing, but instead due to use, corrosion or
damage
-In the case of firearms, the individual characteristics are transferred by the firearm
components (barrel, firing pin, extractor, ejector) to the bullet or cartridge case
Breach Face Marks
-These are formed by high pressure when the bullet is discharged, since it will have a
recoil pressure backwards towards the shooter, which will drive the cartridge back
and against the breech face
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Document Summary
Descriptor factors is what we look for in evidence, a set of little pieces of information in order to be able to identify or compare evidence. These are individual factors that can help to individually distinguish the source of the bullet (which gun) Impression action marks occur when a cartridge case is red in a rearm, and the two most common marks are ring pin impressions and breech marks, and sometimes ejector marks. Note that ejector marks can also be in the form of striated action marks or impression action marks. Striated action marks occur on cartridge cases that have passed through the action of an auto loading or repeating rearm, they are scraping essentially and are produced by coming in contact with: Chamber, shear, ring pin drag marks, extractor marks, and ejector marks. Ejector marks can be in the form of impression or striated action marks.
