Free Scene of the Crime by Anne Wingate Page B

case of an automatic or semiautomatic weapon, empty shells—usually referred to by police as brass— are ejected, and can be located unless the perp takes them away. Once located, the shells can be matched to the weapon (once the weapon has been located) by the firing pin mark on the shell and the ejector mark on the shell. Once again, photographing and triangulating are very important, as once the weapon is located, test firings that demonstrate how far, and in what direction, empties are ejected may—like triangulation of where the slug wound up—determine exactly where the person who fired the weapon was standing.
    The empties should be collected very carefully. Albeit rarely, it has happened that a sufficiently large fragment of fingerprint for identification has been found on an ejected shell.
    Ammo at the Scene
    When searching a crime scene in which the perp probably lived in the house or in which the perp may have grabbed a weapon that was already in the house, it is critical to locate and collect every
    source of ammunition in that house, so that shells can, if possible, be matched to the source.
    The Expert In-and Out of-the Lab
    What can be determined, and by whom, about guns and ammunition?
    • Firearms experts can usually—depending on the condition of the slug—determine what make, model and caliber of weapon fired any given slug.
    • Firearms experts can usually—again depending on the condition of the slug—determine whether a suspect weapon fired any given slug.
    • Firearms experts cannot usually determine which shotgun fired which load of shot.
    • Almost always—except in the rare instances in which a shell has been reloaded and refired several times—firearms examiners can determine which weapon fired which empty shell. This refers to handguns, rifles and shotguns.
    • Using a gunpowder residue test, forensic chemists can usually determine whether or not a given person fired a gun within the last few hours, provided the test is used quickly enough.
    Firearm and Trace Metal Residue Tests
    However, the old paraffin test, which was extremely inaccurate, has been totally discredited. Its technique of coating the hands with warm (not hot) molten paraffin, letting the paraffin harden, then stripping it off and checking it chemically for nitrates (the most common chemicals released when a gun is fired) not only gave false negatives when an insufficient quantity of gunpowder residue was present, it also gave false positives if the suspect had been working with fertilizer or even changing a baby's diaper.
    The best test is a neutron activation analysis (NAA), but this test is so expensive that it is not always used. The test is done on swabs dipped in a 5 percent solution of nitric acid and wiped over the suspect's hands in a given order with particular concentration on the palms of the hands and the webbing between the thumb and first finger, where gunpowder residue tends to collect in the creases. The NAA tests for the less-common barium and antimony contained in gunpowder. Fortunately, the same chemicals and techniques used in the field for the NAA are also used for less expensive chemical tests, so the investigator does not have to decide at once which is to be used.
    However, the better the weapon, the less powder it is likely to discharge onto the shooter's hands. A really well constructed weapon may produce a negative residue test unless an actual NAA is used, rather than its chemical stand-in; a Saturday night special may discharge so much powder that "powder tattooing" may be visible for several days on the webbing between the forefinger and the thumb. (It may also begin to shave lead after it's been fired several times. "Shaving lead" means that the barrel is so far out of line with the cylinder that although most of the slug goes out the barrel, a thin sliver of it is propelled backwards toward the shooter's hand. This happened to me once, when I was test-firing a cheap .22 pistol. The