l8 Forensic Chemistry; Firearms and Toolmarks Flashcards
firearms identification
A discipline mainly concerned with
determining whether a bullet or
cartridge was fired by a particular
weapon; it is not to be confused
with ballistics, which is the study of
a projectile in motion.
grooves
The cut or low-lying portions between the lands in a rifled bore.
rifling
The spiral grooves formed in the bore of a firearm barrel that impart spin to the projectile when it is fired.
bore
The interior of a firearm barrel.
lands
The raised portion between the grooves in a rifled bore.
caliber
The diameter of the bore of a
rifled firearm; the caliber is usually
expressed in hundredths of an inch
or millimeters—for example, .22
caliber and 9 mm.
gauge
Size designation of a shotgun,
originally the number of lead balls
with the same diameter as the
barrel that would make a pound;
for example, a 12-gauge shotgun
would have a bore diameter of a
lead ball 1/12 pound in weight; the
only exception is the .410 shotgun,
in which bore size is 0.41 inch.
breechface
The rear part of a firearm barrel.
extractor
The mechanism in a firearm by
which a cartridge of a fired case is
withdrawn from the chamber.
ejector
The mechanism in a firearm that
throws the fired cartridge case
from the firearm.
distance determination
The process of determining the distance between the firearm and a target is usually based on the distribution of powder patterns or the spread of a shot pattern.
choke
An interior constriction placed at or
near the muzzle end of a shotgun’s
barrel to control shot dispersion
Greiss test
A chemical test used to develop patterns of gunpowder residues
around bullet holes.
what is a tool mark?
Any impression, cut, gouge, or abrasion caused by a tool coming into contact with another object
careful examination of a tool mark impression can reveal..? and the presence of minute imperfections can..?
important class characteristics, impart individuality
what are 4 common tasks of a forensic firearms laboratory?
- Comparison of bullets and casings
- Detection and characterization of gunpowder residues
-Garments, wounds, and hands - Estimation of muzzle-to-target distances
- Restoration of obliterated serial numbers on weapons
briefly describe handguns
-designed to be fired with one hand
-common types: single shot, revolver and semi-auto
-can be single-action or double-action
briefly describe long guns
-Designed to be fired while resting on the shoulder
-Can be single-shot, repeating, semi-automatic, or automatic
-shotguns and rifles
briefly describe shotguns
-smooth barrels
-Shell ammunition contains numerous ball-shaped projectiles called shot
briefly describe rifles
-barrels have lands and grooves
-Bullet ammunition is impressed with lands and grooves during firing
list 4 parts of a gun that can leave ‘toolmarks’ on a bullet when fired
-barrel
-firing pin
-breechface
-ejector
explain class characteristics of a gun
markings are peculiar to a particular type of gun
explain how individual characteristics of a gun can be used to link suspects to crime
-bullet acquires features of the barrel as it is fired
If one bullet at the scene of a crime and another test-fired from a suspect’s gun show the same markings
–the suspect is linked to the crime
what is the main source of both class and individual evidence of a gun?
the barrel
list type and examples of characteristics from the rifling method?
defines class characteristics of a barrel
- # of lands and grooves
- width (caliber)
- direction of twist (left or right handed)
explain striations of a gun?
-fine lines found in the interior of the barrel due
to imperfections in rifling methods, Imparted on a bullet as it passes through
-microscopic and randomly irregular
-unique to each barrel
-form individual characteristics
what is true of striations?
No two rifled barrels, even those manufactured in succession, have identical striation markings
how are bullet markings compared?
-examiner must obtain “test bullets” fired through the
suspect barrel for comparison
-Initial stages of examination look at class characteristics
-Any differences at this stage will exclude this barrel
-same class characteristics, move onto matching striations with a comparison microscope
3 things that exit a firearm when fired
-Bullet
-Smoke
-Unburned and partially burned gunpowder
what allows for distance determination?
distribution of gunpowder particles and other discharge residues around a bullet hole
how can an examiner assess distance?
comparison of powder-residue patterns from:
-Patterns on victim’s clothing or skin
-Test patterns generated from suspect weapon
how can we determine whether or not an individual has recently fired a weapon?
-Gunpowder and primer residues are also blown back
toward the shooter
-often on the firing hand
-detection of residues can provide info
