Ballistics In Forensic Science Pdf
Mertie Oldow
Forensic firearm examination is the forensic process of examining the characteristics of firearms or bullets left behind at a crime scene. Specialists in this field try to link bullets to weapons and weapons to individuals. They can raise and record obliterated serial numbers in an attempt to find the registered owner of a weapon and look for fingerprints on a weapon and cartridges.
By examining unique striations impressed into a bullet from the barrel of a gun, expended ammunition can be linked back to a specific weapon.[1] These striations are due to the rifling inside the barrels of firearms. Rifling spins the bullet when it is fired out of the barrel to improve precision.[2] Although bullet striations are individualized unique evidence, microscopic striations in the barrel of the weapon are subject to change slightly, after each round that is fired.[1] For this reason, forensic ballistics examiners may not fire more than five shots from a weapon found at a scene.[3] Known exemplars taken from a seized weapon can be compared to samples recovered from a scene using a comparison microscope as well as newer 3-D imaging technology. Striation images can also be uploaded to national databases. Furthermore, the markings can be compared to other images in an attempt to link one weapon to multiple crime scenes.
Like all forensic specialties, forensic firearm examiners are subject to being called to testify in court as expert witnesses. However, the reliability of some techniques of forensic firearm examination have been criticized.[4][5]
The ability to compare ammunition is a direct result of the invention of rifling around the turn of the 16th century.[6] By forcing the bullet to spin as it travels down the barrel of the weapon the precision is greatly increased. At the same time, the rifling leaves marks on the bullet that are indicative of that particular barrel. Prior to mass production of firearms, each barrel and bullet mold was hand made by gunsmiths making them unique.[7] The first successful documented case of forensic firearm examination occurred in 1835 when a member of the Bow Street Runners in London matched a recovered bullet from a murder victim to a specific mold in a suspect's home confirming that he made the bullet; this gave further evidence that the bullet maker was the perpetrator and he was convicted.[6] As manufacturing and automation replaced hand tools, the ability to compare bullets became impossible due to the standardization of molds within a specific company. However, experts in the field postulated that there were microscopic differences on each barrel left during the manufacturing process. These differences were a result of wear on the machines and since each new weapon caused a tiny amount of wear, each barrel would be slightly different from every other barrel produced by that company.[7] Also, each bullet fired from a specific barrel would be printed with the same marks, allowing investigators to identify the weapon that fired a specific bullet.[8]
One of the first uses of this knowledge was in 1915 to exonerate Charles Stielow of the murder of his neighbors. Stielow was sentenced to death and appealed to Charles S. Whitman, the Governor of New York, who was not convinced by the evidence used to convict Stielow. Whitman halted the execution until an inquiry could be conducted and after further examination it was shown that Stielow's firearm could not have fired the bullets recovered from the victims.[9] The invention of the comparison microscope by Calvin Goddard and Phillip O. Gravelle in 1925 modernized the forensic examination of firearms.[10] Simultaneous comparison of two different objects at the same time allowed to closely examine striations for matches and therefore make a more definitive statement as to whether or not they matched.
One of the first true tests of this new technology was in the aftermath of the Saint Valentine's Day Massacre in 1929. During the Prohibition Era, competing gang members were fighting over bootlegging operations within the city of Chicago. Members of the Chicago Outfit and the Egan's Rats led by Al Capone attempted to remove all competition from Chicago by eliminating the North Side Gang leader Bugs Moran.[11][12] The massacre missed Moran, who was not present, but killed seven members of the North Side Gang. The murderers attempted to cover up their crime by posing as police officers, even dressing in police uniforms.[12] Witnesses saw two "officers" leaving the scene, which implicated the Chicago police department as the perpetrators of the massacre. High levels of police corruption during that time period made it seem likely that the police department committed the killings.[12] The investigation stalled until December 1929 when Fred Burke, a member of the Egan's Rats, shot and killed a police officer in St. Joseph, Michigan. Officers searching for Burke were led to a home in nearby Stevensville. While Burke was not there, inside officers found an arsenal of weapons including two Thompson submachine guns.[12] The Chicago police department was contacted and the weapons were brought back to Chicago for testing. Goddard was asked to compare the weapons to collected evidence found at the massacre using his new "ballistic-forensics" technique. After test firing the guns, Goddard proved that the weapons were those used to kill the members of the North Side Gang, absolving the Chicago police department of all involvement.[12] The successful use of Goddard's technique resulted in the solidification of his place as the father of forensic firearm examination.[13]
Any firearm collected during the course of an investigation could yield viable evidence if examined. For forensic firearm examination specific evidence that can be recovered include weapon serial numbers and potentially fingerprints left on the weapon's surface.
Fingerprint recovery from the surface of firearms is done with cyanoacrylate (more commonly known as superglue) fuming.[14] Firearms are placed in a specially designed fume hood designed to evenly distribute fumes instead of removing them. Liquid superglue is placed in a container and heated until it is in a gaseous state. The circulating fumes adhere to the oils left behind by the fingerprint, turning the print white.[15] The resulting white print can be enhanced with fingerprint powder to increase the contrast of the white print against the weapon's finish.[14] While using the fuming technique on recovered guns is commonplace, the recovery of fingerprints from the surfaces of a firearm is challenging due to the textured grip and the general condition of recovered weapons.[14][16] If fingerprints are recovered, they can be processed through fingerprint databases such as the Integrated Automated Fingerprint Identification System (IAFIS). Various parts of the recovered weapon can also be tested for touch DNA left by whoever handled it. However, the low levels of DNA that can be recovered presents numerous issues such as contamination and analysis anomalies such as allele drop-out and drop-in.[17]
If the serial number is successfully restored it can be used to help investigators track the weapon's history, as well as potentially determine who owns the weapon.[23] Firearm databases such as the National Crime Information Center of the United States and INTERPOL's Firearm Reference Table can be used by investigators to track weapons that have been lost, stolen, or used previously in other crimes.[24][25]
Originally developed as a method to detect flaws or irregularities in magnetic materials, magnetic particle inspection can be used on firearms to visualize the serial number underneath the obliterated area.[22] When performing this technique, examiners place the weapon in a magnetic field. The irregularities in the metal, in this case the serial number, cause the field to deform.[22] When a solution of ferrous particles is added to the weapon's magnetized surface they will be attracted to the area where the magnetic field has deformed and will build up in the area.[26] If fluorescent particles are added to the ferrous solution, ultraviolet light can be used to make it easier to visualize any recovered serial number.[26]
Chemical restoration is a type of chemical milling. Typically, chemical milling is used to slowly remove material to create a desired shape. In serial number restoration, small amounts of metal are removed until variations in the metal corresponding to the serial number are visible. This is possible because stamping the numbers distorts the grain boundary structure underneath the surface of the metal. However, chemical restoration is limited to that depth and is only successful when the obliteration of the serial number is superficial.[27] Examiners performing a restoration first sand the area where the serial number used to be. This removes any debris from the area left when the serial number was obliterated.[28] The examiner then chooses a chemical, usually an acid,[28] that will be used to slowly bring the number back to the surface. The type of chemical that is used depends on the material the weapon is made of. These acids can range from Fry's Reagent for a magnetic metal,[21] which is a mixture of hydrochloric acid, cupric chloride, and distilled water,[29] to an acidic ferric chloride solution for a non-magnetic, non-aluminum material.[21]
Cartridges are also routinely examined for fingerprints as the act of loading the ammunition into the magazine, or chamber, leaves recoverable impressions. These fingerprints can survive the firing processes and, while a rare occurrence, fingerprints have been obtained from cartridges recovered from the scene.[31] Cartridges are subjected to cyanoacrylate fuming and examined for any usable prints. Usable prints are photographed and can be uploaded to fingerprint databases such as IAFIS for comparison with known exemplars. Cartridges can also be swabbed for trace DNA left by the individual who loaded the magazine. The extremely low levels of recoverable DNA present the same issues as swabbing a firearm for DNA.[17]
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