Firearm and Toolmark Evidence Flashcards
- Four universal firearm safety rules
o Treat all firearms as if they are loaded
o Never point the muzzle at anything you do not intend to destroy
o Never put your finger on the trigger until ready to fire
o Know your backstop
what happens when two objects interact?
- When two objects interact with each other the harder object will leave marks on the softer object. The harder object is the Tool
- Tool working surface =
the area of the tool that contacts the softer object to leave its mark
- Class characteristics
measurable features of a specimen which indicate a restricted group source. They result from design factors, and are therefore determined prior to manufacture
- Individual characteristics =
Marks produced by the random imperfections or irregularities of tool surfaces. These random 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 other tools with the same class characteristics
- Subclass characteristics
More restrictive group source than class characteristics that is also produced incidental to manufacturing. Characteristics that appear individual; however, they arise as carryover in the manufacturing process
- Tool marks can be impressed marks or striated marks
o Impressed marks are marks produced when a tool is placed against another object and enough force is applied to the tool so that it leaves an impression
o Striated marks are marks produced when a tool is placed against another object and with pressure applied, the tool is moved across the object producing a striated mark
can impressed marks and striated have individual characteristics?
Impressed marks and striated marks may have individual characteristics, especially if the tool used is worn or damaged
- Tool mark analysis?
o Need to establish the physical action necessary to make the questioned mark
o Using the tool, reproduce the mark in a softer material such as lead
o Compare the question mark with the known mark using a comparison microscope
- Comparison microscope
o Allows for a bullet, cartridge case, and other toolmark comparisons
o Two separate stages
o Images side by side in microscope eyepiece
o Usually has a trinocular for a camera mount
- Rifling
o The inner surface of the barrel is cut or impressed with spiral grooves
o Lands are the raised areas
o The rifling grooves engage the bullet and impart spin
o The spin stabilizes the bullet in flight
- Gun barrel class characteristics
o Direction of twist (left or right)
o Land width (measured in thousandths of an inch)
o Groove width (measured in thousandths of an inch)
o Number of lands and grooves
o Caliber (measure of distance in inches from land to opposite land)
- Gun barrel individual characteristics
o Striations within the lands and grooves
o Land impressions are the most individual
o No two rifled barrels will have identical striated markings
- Bullet comparisons
o Gun barrel marks a bullet as it travels down barrel
o Markings constitute class and individual characteristics
o Result: bullet can be associated to a particular firearm
- Caliber
o Firearms=The diameter of the barrel measured between two opposite lands
o Bullets=The diameter of the base of the bullet
o Three caliber naming conventions
* True caliber=actual measurement
* Nominal caliber=caliber family (e.g. nominal 38 caliber includes 9mm, 357 mag, 38 special, 380 Auto) Nominal caliber is not an actual measurement as in true caliber
* Cartridge designation=Manufacturers description of the cartridge and can be synonymous with nominal caliber
- Cartridge cases and cycle of fire
o Firing pin strikes primer
o Primer ignites powder charge
o Gases are evolved
o Bullet is forced down the barrel
o Cartridge case obturates
o For auto and semi-autos the cartridge case is extracted and ejected and another cartridge is fed into the chamber
- Source of markings found on cartridge cases
o Firing pin
o Firing pin aperture
o Breachface
o Chamber wall
o Ejection port
o Magazine lip
o Extractor
o Ejector
- Shotguns
o Have smooth bore (no rifling)
o The wad can reveal the gauge and manufacturer of the ammo
o The smaller the gauge number the larger the bore diameter
- Microscopic comparisons
o Identifications can be made when all class characteristics and a sufficient amount of individual characteristics agree with each other
o Eliminations can be made when one or more class characteristics don’t agree with each other and/or an insufficient amount of individual characteristics agree with each other
o Inconclusive is made when all class characteristics agree with each other, but an insufficient amount of individual characteristics agree with each other
o Unsuitable result is due to post-fire abrasions that destroy microscopic detail or fragment that does not contain any microscopic detail (rifling)
- General rifling characteristics (GRC) database
o Maintained and updated by the FBI
o Caliber information
o Number of lands and grooves
o Direction of twist
o Land and groove widths
o Firing pin shape
o Type of rifling
- Partially burned and unburned gun powder contains:
o Lead
o Nitrites
* Lead and Nitrites are main two components for distance determinations
o Antimony
o Barium
o Others
- Contact Wounds
o Stellate pattern/ firearm imprint
o Little or no powder residue around wound
o Charring or melting of fabric on clothing
- Greater than Contact Wounds
o Partially burnt powder deposits around wound
o Powder density pattern will indicate distance
- Gun Shot Residue (GSR) testing
o Sodium Rhodizinate = test for lead
o Greiss test = test for nitrites (partially burnt gun powder)
o Greiss test must be done before Sodium Rhodizinate test (chemical interference)
