Collecting+Evidence+and+Laboratory+Examination

=Collecting Evidence and Laboratory Examination=

Vocabulary
 * 1) Birefringent – Having two or more indices of refraction. When place between polarizing filters, birefringent materials exhibit bands of color.
 * 2) Bolt-face signatures – Marking embossed on a cartridge primer (or base of cartridge) by a breechblock or bolt
 * 3) Bullet wipe – Soot, lubricant, or other material wiped from the surface of a bullet onto skin or other surfaces penetrated by the bullet.
 * 4) Diluent – Inert substance used to increase the mass of a controlled substance; exerts no physiological effect; is used to give the illusion that more controlled substance is present than actually present.
 * 5) Forcing cone – The flaring at the breech end of the barrel of a revolver. It serves to guide the bullet into the rifling.
 * 6) Schlieren optics – Imaging system in which the transparent or translucent object to be examined is place between two spherical mirrors.
 * 7) Skid marks – Mark on the surface of a fired bullet made when the edges at the beginning of the rifling scrape the bullet surface or when the nose of a bullet slides on the surface of the forcing cone of a revolver barrel.
 * 8) Slippage – Mark on the surface of a fired bullet made when the bullet slides along the tops of the lands of the rifling.
 * 9) Stellate defect – Star-like tearing of soft tissue seen in contact wounds of the head or sternum.
 * 10) Yaw angle – Angle between the axis of a bullet and its trajectory.


 * The National Integrated Ballistic Information Network (NIBIN):** Developed to facilitate linking firearm evidence in cases in different jurisdictions.
 * John Davis:** In the 1950s, he developed the striagraph, which used a system of levers to amplify the movement of a stylus as it was drawm over the surface of a fired bullet. The Striagraph produced a circular graph that reflected the profile of the bullet surface.
 * Cartridge Headstamp:** Bears the information such as the vendor of the ammunition and the gauge or caliber of the cartridge.


 * Microscopic Examination:**
 * Bullets/weapons should be recovered from crime scene and their serial numbers should be documented, all removable parts should be marked for identification and the weapon should be unloaded for safety
 * 9mm semiautomatic pistol should be marked on the barrel, the slide, the receiver, and the magazine
 * Bullets and weapons should be examined for the prescence of trace evidence as well as patterened markings.
 * Bullets may pick up textile fibers, traces of paint, or bits of concrete and brick from intermediate targets
 * Then they determine the general rifling characteristics
 * Diameter of the fired bullet is measured with a micrometer or compared to fired standards to determine a caliber if the bullet is not deformed
 * If the bullet is deformed, it can be weighed to determine the caliber
 * If the bullet is fragmented, it is possible to determine calibers by combining the measured width of a land marking with that of an adjacent groove
 * Class characteristics are then determined by inspection
 * The forensic examiner will obtain test-fired bullets by firing the suspected firearm into a bullet trap
 * The test fire bullets are compared under a microscope with the bullet/bullets from the crime scene
 * Matches are determined by comparing the striations on the test fired bullet with the crime scene bullet
 * They draw their conclusion



> Degree of twist of the rifling These are the goals of the initial examination of a fired bullet
 * General Rifling Characteristics to be Determined:**
 * Caliber
 * Number of lands and grooves
 * Direction of twist of the rifling
 * Widths of lands and grooves

Used to serch the FBI database for matching makes and models of firearms
 * Class Characteristics of Firearms:**
 * Caliber
 * Shape of firing chamber
 * Location of the firing pin
 * Size and shape of the firing pin
 * Size of extractors and ejectors (if any)
 * Geometrical relationship of the extractor and ejector


 * Conclusions based off the comparirson of bullets or cartridges:**
 * The questioned bullet or cartridge was fired in the submitted weapon. This means that the class characteristics are consisten and that individual characteristics match. This is a positive identification.
 * The questioned bullet or cartridge was not fired in the submitted weapon. This means that the class characteristics did not match. In this instance, no microscopical comparison will have been undertaken. This is a negative indentification.
 * The results of the microscopical examination were inconclusive. This means that the class characteristics match, but that insufficient individual characteristics to declare a match could not be found. The firearms examiner does not render a negative identification in this instance because the individual characteristics of the firearm may have been altered by further use. For example, there may have been bore erosion due to firing a large number of round through the weapon. The weapon may also have been deliberately altered by the shooter. Some weapons (e.g. Glock pistols) do not make usable striation patterns on fired bullets.