<CHAP NUM="15" ID="CH.00.015">chapter 15
<FM><TTL>Firearms, Tool Marks, and Other Impressions</TTL>
<KTSET><TTL>Key Terms</TTL>
<KT>bore</KT>
<KT>breechblock</KT>
<KT>caliber</KT>
<KT>choke</KT>
<KT>distance determination</KT>
<KT>ejector</KT>
<KT>extractor</KT>
<KT>firearms identification</KT>
<KT>gauge</KT>
<KT>Greiss test</KT>
<KT>grooves</KT>
<KT>lands</KT>
<KT>rifling</KT></KTSET>
<OBJSET><TTL>Learning Objectives</TTL>
<P>After studying this chapter you should be able to:
<OBJ><P><INST>< </INST>Describe techniques for rifling a barrel</P></OBJ>
<OBJ><P><INST>< </INST>Recognize the class and individual characteristics of bullets and cartridge cases</P></OBJ>
<OBJ><P><INST>< </INST>Understand the use of the comparison microscope to compare bullets and cartridge cases</P></OBJ>
<OBJ><P><INST>< </INST>Explain the concept of the NIBIN database</P></OBJ>
<OBJ><P><INST>< </INST>Explain the procedure for determining how far a weapon was fired from a target</P></OBJ>
<OBJ><P><INST>< </INST>Identify the laboratory tests for determining whether an individual has fired a weapon</P></OBJ>
<OBJ><P><INST>< </INST>Explain the forensic significance of class and individual characteristics to the comparison of toolmark, footwear, and tire impressions</P></OBJ>
<OBJ><P><INST>< </INST>List some common field reagents used to enhance bloody footprints</P></OBJ></P></OBJSET></FM>
<CASE NUM="1" TY="CS"><TTL>The Beltway Snipers</TTL>
<P>During a three-week period in October 2002, ten people were killed and three others were wounded as two snipers terrorized the region in and around the Baltimore–Washington metropolitan area. The arrest of John Allen Muhammad, 41, and Lee Boyd Malvo, 17, ended the ordeal. The semiautomatic .223-caliber rifle seized from them was ultimately linked by ballistic tests to eight of the ten killings. The car that Muhammad and Malvo were driving had been specially configured with one hole in the trunk through which a rifle barrel could protrude, so that a sniper could shoot from inside a slightly ajar trunk.</P>
<P>The major break in the case came when a friend of Muhammad’s called police suggesting that Muhammad and his friend Malvo were the likely snipers. Muhammad’s automobile records revealed numerous traffic stops in the Beltway area during the time of the shootings. Another break in the case came when Malvo called a priest to boast of a killing weeks before in Montgomery, Alabama. Investigators traced the claim to a recent liquor store holdup that left one person dead. Fortunately, the perpetrator of this crime left a latent fingerprint at the murder scene. Authorities quickly tracked the print to Malvo, a Jamaican citizen, through his fingerprints on file with the Immigration and Naturalization Service. A description of Muhammad’s car was released to the media, leading to tips from alert citizens who noticed the car parked in a rest area with both occupants asleep.</P>
<P>The motive for the shooting spree was believed to be a planned plot to extort $10 million from local and state governments. Muhammad was sentenced to death and Malvo is currently serving life imprisonment without parole.</P></CASE>
<BM><P>Just as natural variations in skin ridge patterns and characteristics provide a key to human identification, minute random markings on surfaces can impart individuality to inanimate objects. Structural variations and irregularities caused by scratches, nicks, breaks, and wear permit the criminalist to relate a bullet to a gun; a scratch or abrasion mark to a single tool; or a tire track to a particular automobile. Individualization, so vigorously pursued in all other areas of criminalistics, is frequently attainable in firearms and tool mark examination.</P>
<P>Although a portion of this chapter will be devoted to the comparison of surface features for the purposes of bullet identification, a complete description of the services and capabilities of the modern forensic firearms laboratory cannot be restricted to just this one subject, important as it may be. The high frequency of shooting cases means that the science of <KT>firearms identification</KT><SIDEIND NUM="1" ID="MN2.15.001"/> must extend beyond mere comparison of bullets to include knowledge of the operation of all types of weapons, restoration of obliterated serial numbers on weapons, detection and characterization of gunpowder residues on garments and around wounds, estimation of muzzle-to-target distances, and detection of powder residues on hands. Each of these functions will be covered in this chapter.</P>
<H1><SCAP></SCAP>Bullet Comparisons</H1>
<P>The inner surface of the barrel of a gun leaves its markings on a bullet passing through it. These markings are peculiar to each gun. Hence, if one bullet found 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. Because these inner surface striations are so important for bullet comparison, it is important to know why and how they originate.</P>
<P>The gun barrel is produced from a solid bar of steel that has been hollowed out by drilling. The microscopic drill marks left on the barrel’s inner surface are randomly irregular and in themselves impart a uniqueness to each barrel. However, the manufacture of a barrel requires the additional step of impressing its inner surface with spiral <KT>grooves</KT><SIDEIND NUM="2" ID="MN2.15.002"/>, a step known as <KT>rifling</KT><SIDEIND NUM="3" ID="MN2.15.003"/>. The surfaces of the original <KT>bore</KT><SIDEIND NUM="4" ID="MN2.15.004"/> remaining between the grooves are called <KT>lands</KT><SIDEIND NUM="5" ID="MN2.15.005"/> (see <LINK LINKEND="FG.15.001">Figure <FIGIND NUM="1" ID="FG.15.001"/>15–1</LINK>). As a fired bullet travels through a barrel, it engages the rifling grooves; these grooves then guide the bullet through the barrel, giving it a rapid spin. This is done because a spinning bullet does not tumble end over end on leaving the barrel, but remains instead on a true and accurate course.</P>
<P>The diameter of the gun barrel, sketched in <LINK LINKEND="FG.15.002">Figure <FIGIND NUM="2" ID="FG.15.002"/>15–2</LINK>, measured between opposite lands, is known as the <KT>caliber</KT><SIDEIND NUM="6" ID="MN2.15.006"/> of the weapon. Caliber is normally recorded in hundredths of an inch or in millimeters—for example, .22 caliber and 9 mm. Actually, the term <ITAL>caliber,</ITAL> as it is commonly applied, is not an exact measurement of the barrel’s diameter; for example, a .38-caliber weapon might actually have a bore diameter that ranges from 0.345 to 0.365 inch.</P>
<P>Before 1940, barrels were rifled by having one or two grooves at a time cut into the surface with steel hook cutters. The cutting tool was rotated as it passed down the barrel, so that the final results were grooves spiraling either to the right or left. However, as the need for increased speed in the manufacture of weapons became apparent, newer techniques were developed that were far more suitable for the mass production of weapons. The broach cutter, shown in <LINK LINKEND="FG.15.003">Figure <FIGIND NUM="3" ID="FG.15.003"/>15–3</LINK>, consists of a series of concentric steel rings, with each ring slightly larger than the preceding one. As the broach passes through the barrel, it simultaneously cuts all grooves into the barrel at the required depth. The broach rotates as it passes through the barrel, giving the grooves their desired direction and rate of twist.</P>
<P>In contrast to the broach, the button process involves no cuttings. A steel plug or “button” impressed with the desired number of grooves is forced under extremely high pressures through the barrel. A single pass of the button down the barrel compresses the metal to create lands and grooves on the barrel walls that are negative forms of those on the button. The button rotates to produce the desired direction and rate of twist (see <LINK LINKEND="FG.15.004">Figure <FIGIND NUM="4" ID="FG.15.004"/>15–4</LINK>).</P>
<P>Like the button process, the mandrel rifling or hummer forging process involves no cutting of metal. A mandrel is a rod of hardened steel machined so its form is the reverse impression of the rifling it is intended to produce. The mandrel is inserted into a slightly oversized bore, and the barrel is compressed with hammering or heavy rollers into the mandrel’s form.</P>
<P>Every firearms manufacturer chooses a rifling process that is best suited to meet the production standards and requirements of its product. Once the choice is made, however, the class characteristics of the weapon’s barrel will remain consistent; each will have the same number of lands and grooves, with the same approximate width and direction of twist. For example, .32-caliber Smith & Wesson revolvers have five lands and grooves twisting to the right. On the other hand, Colt .32-caliber revolvers exhibit six lands and grooves twisting to the left. Although these class characteristics permit the examiner to distinguish one type or brand name of weapon from another, they do not impart individuality to any one barrel; no class characteristic can do this.</P>
<P>If one could cut a barrel open lengthwise, a careful examination of the interior would reveal the existence of fine lines, or <ITAL>striations</ITAL>, many running the length of the barrel’s lands and grooves. These striations are impressed into the metal as the negatives of minute imperfections found on the rifling cutter’s surface, or they are produced by minute chips of steel pushed against the barrel’s inner surface by a moving broach cutter. The random distribution and irregularities of these markings are impossible to duplicate exactly in any two barrels. <BOLD>No two rifled barrels, even those manufactured in succession, have identical striation markings.</BOLD> These striations form the individual characteristics of the barrel.</P>
<P>As the bullet passes through the barrel, its surface is impressed with the rifled markings of the barrel. The bullet emerges from the barrel carrying the impressions of the bore’s interior surface; these impressions reflect both the class and individual characteristics of the barrel (see <LINK LINKEND="FG.15.005">Figure <FIGIND NUM="5" ID="FG.15.005"/>15–5</LINK>). Because there is no practical way of making a direct comparison between the markings on the fired bullet and those found within a barrel, the examiner must obtain test bullets fired through the suspect barrel for comparison. To prevent damage to the test bullet’s markings and to facilitate the bullet’s recovery, test firings are normally made into a recovery box filled with cotton or into a water tank.</P>
<P>The number of lands and grooves, and their direction of twist, are obvious points of comparison during the initial stages of the examination. Any differences in these class characteristics immediately eliminate the possibility that both bullets traveled through the same barrel. A bullet with five lands and grooves could not possibly have been fired from a weapon of like caliber with six lands and grooves, nor could one having a right twist have come through a barrel impressed with a left twist. If both bullets carry the same class characteristics, the analyst must begin to match the striated markings on both bullets. This can be done only with the assistance of the comparison microscope (see <OLINK LOCALINFO="CH.00.007">Chapter 7</OLINK>).</P>
<P>Modern firearms identification began with the development and use of the comparison microscope. This instrument is the most important tool at the disposal of the firearms examiner. The test and evidence bullets are mounted on cylindrical adjustable holders beneath the objective lenses of the microscope, each pointing in the same direction (see <LINK LINKEND="FG.15.006">Figure <FIGIND NUM="6" ID="FG.15.006"/>15–6</LINK>). Both bullets are observed simultaneously within the same field of view, and the examiner rotates one bullet until a well-defined land or groove comes into view. Once the striation markings are located, the other bullet is rotated until a matching region is found. Not only must the lands and grooves of the test and evidence bullet have identical widths, but the longitudinal striations on each must coincide. When a matching area is located, the two bullets are simultaneously rotated to obtain additional matching areas around the periphery of the bullets. <LINK LINKEND="FG.15.007">Figure <FIGIND NUM="7" ID="FG.15.007"/>15–7</LINK> shows a typical photomicrograph of a bullet match as viewed under a comparison microscope.</P>
<P>Unfortunately, the firearms examiner rarely encounters a perfect match all around the bullet’s periphery. The presence of grit and rust can alter the markings on bullets fired through the same barrel. More commonly, recovered evidence bullets may become so mutilated and distorted on impact as to yield only a small area with intact markings. Furthermore, striation markings on a barrel are not permanent structures; they are subject to continuing change and alteration through wear as succeeding bullets traverse the length of the barrel. Fortunately, in most cases, these changes are not dramatic and do not prevent the matching of two bullets fired by the same weapon. As with fingerprint comparison, there are no hard-and-fast rules governing the minimum number of points required for a bullet comparison. The final opinion must be based on the judgment, experience, and knowledge of the expert.</P>
<P>Frequently, the firearms examiner receives a spent bullet without an accompanying suspect weapon and is asked to determine the caliber and possible make of the weapon. If a bullet appears not to have lost its metal, its weight may be one factor in determining its caliber. In some instances, the number of lands and grooves, the direction of twist, and the widths of lands and grooves are useful class characteristics for eliminating certain makes of weapons from consideration. For example, a bullet that has five lands and grooves and twists to the right could not come from a weapon manufactured by Colt, because Colts are not manufactured with these class characteristics. Sometimes a bullet has rifling marks that set it apart from most other manufactured weapons, as in the case of Marlin rifles. These weapons are rifled by a technique known as <ITAL>microgrooving</ITAL> and may have eight to twenty-four grooves impressed into their barrels; few other weapons are manufactured in this fashion. In this respect, the FBI maintains a record known as the General Rifling Characteristics File. This file contains listings of class characteristics, such as land and groove width dimensions, for known weapons. It is periodically updated and distributed to the law enforcement community to help identify rifled weapons from retrieved bullets.<SIDEIND NUM="101" ID="MN1.15.101"/></P>
<P>Unlike rifled firearms, a shotgun has a smooth barrel. It therefore follows that projectiles passing through a shotgun barrel are not impressed with any characteristic markings that can later be related back to the weapon. Shotguns generally fire small lead balls or pellets contained within a shotgun shell (see <LINK LINKEND="FG.15.008">Figure <FIGIND NUM="8" ID="FG.15.008"/>15–8</LINK>). A paper or plastic wad pushes the pellets through the barrel on ignition of the cartridge’s powder charge. By weighing and measuring the diameter of the shot recovered at a crime scene, the examiner can usually determine the size of shot used in the shell. The size and shape of the recovered wad may also reveal the gauge of the shotgun used and, in some instances, may indicate the manufacturer of the fired shell.</P><SIDEIND NUM="102" ID="MN1.15.102"/>
<P>The diameter of the shotgun barrel is expressed by the term <KT>gauge</KT>.<SIDEIND NUM="7" ID="MN2.15.007"/><FNIND NUMBER="1"/>1 The higher the gauge number, the smaller the barrel’s diameter. For example, a 12-gauge shotgun has a bore diameter of 0.730 inch as contrasted to 0.670 inch for a 16-gauge shotgun. The exception to this rule is the .410-gauge shotgun, which refers to a barrel 0.41 inch in diameter.</P>
<H1><SCAP></SCAP>Cartridge Cases</H1><SCAP></SCAP>
<P>The...
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