FOS 405 Firearms & Toolmarks

Ballistics Unit Services

• Identification of evidence

  • type of firearm

  • Make of firearm

  • Caliber of firearm: diameter of the bore, measure btwn lands, hundredths of an inch or mm

• Microscopic comparisons

  • rifling

  • Toolmarks on machined parts

  • Toolmark cases handled by the Trace Evidence Section

  • Possible outcomes (+), (-), or inc.

Ballistics Unit Services cont.

• Serial number restorations: use acids or bases to eat away metal to get underneath

• Distance determinations: how much gunshot residue is around

• Chemical analysis

• Bullet trajectory reconstruction

Ballistics

Study of projectile in motion, 3 types of ballistics

• internal - occurs inside gun

• External - when it leaves gun

• Terminal - upon impact

Firearms identification

discipline concerned w/determining whether a bullet or cartridge was fired b a particular weapon

Gun Barrel Markings

• barrel/bore - produced from a solid bar of steel that has been hollowed out by drilling

• Drilling imparts tiny, randomly irreg markings, unique to each gun

• inner surface of the barrel/bore of gun leaves it markings on a bullet passing through it

• Rifling - spiral grooves formed in the bore of a firearm barrel that impart spin & accuracy to the projectile

Gun Barrel Marking cont.

• broach cutting - broach cuts grooves into barrel

• Button processing - no cutting, steel plug, button compresses metal

• Hammer forging - no cutting, mandrel rod of steel, barrel compressed around mandrel

• Grooves - cut or low-lying portions btwn the lands in a rifled bore. Guide the fired bullet through the barrel

• Lands - surfaces of the original bore remaining btwn the grooves

Caliber

• diameter of the gun barrel

• Once a manufacturer chooses a rifling process the class characteristics of the weapon’s barrel will remain constant

  • same # of lands, grooves

  • Same approx l & g widths

  • Same direction of twist right or left handed