Saferstein Chapter 09 - Firearms, Tool Marks, and Other Impressions PDF

& RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG 1  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJK...

& RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG 1  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG 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. 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. The high frequency of shooting cases means that the science of firearms identification 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 in the past the detection of powder residues on hands. Generally, firearms can be divided into two categories: handguns and long guns. Handguns, or pistols, are firearms that are designed to be held and fired with one hand. The three most common types of handguns are single‐shot handguns, revolvers, and semiautomatic pistols. All handguns can be classified as single‐action or double‐action firearms. Single‐action 2 firearms require the firing component to be manually cocked backward each time before the trigger is pulled in order to fire. Double‐action firearms cock the firing component when the trigger is pulled and then reload the firing chamber after the round is fired. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 2  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Handguns, or pistols, are firearms that are designed to be held and fired with one hand. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 3 & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG The revolver features several firing chambers located within a revolving cylinder. As the revolver is fired, the cylinder can rotate clockwise or counterclockwise. Each firing chamber holds one cartridge, which is lined up with the barrel mechanically when the round is fired. The cartridge cases have to be manually ejected to reload the firing chambers. Swing‐out revolvers feature a cylinder that swings out to the side of the weapon to be loaded. Break‐ top revolvers are hinged so that both the barrel and the cylinder flip downward for loading. Solid‐frame revolvers have no mechanism to uncover all the firing chambers at once. Instead, a small “gate” at the back of the gun allows one chamber to be loaded at a time; the cylinder is then rotated, and the next chamber is loaded with a cartridge. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 4  ' RUOLQJ. LQGHUVOH\* HWW\,P DJHV & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Semiautomatic pistols feature a removable magazine that is most often contained within the grip of the firearm. Once the magazine is loaded, the firing component is cocked by pulling the slide on the top of the gun rearward and then releasing it to load the first round. The firing of the cartridge generates gases that are used to eject the cartridge case, cock the firing component, and load the next round. A semiautomatic pistol fires one shot per trigger pull. An automatic firearm, such as a machine gun, fires as long as the trigger is pressed or until the ammunition is depleted. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 5 & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Long guns are either rifles or shotguns. Rifles and shotguns are designed to be fired while resting on the shoulder. The two principal differences between rifled firearms and shotguns are found in the ammunition and the barrel. Shotgun ammunition, called a shell, contains numerous ball‐shaped projectiles, called slug. The barrel of a shotgun is smooth, without the grooves and lands found in rifles. A shotgun barrel can also be narrowed toward the muzzle in order to concentrate shot when fired. This narrowing of the barrel is called the choke of the shotgun. A shotgun may be single or double barreled. The two barrels of a double‐barreled shotgun may be arranged horizontally (side by side) or vertically (one over another). The barrels may also have different choke diameters. The various types of rifles and shotguns have different reloading mechanisms. The single‐shot gun can chamber and fire only one round at a time. Just as with single‐shot pistols, the round has to be loaded manually each time. Repeating long guns use a mechanical instrument of some sort to eject spent cartridge cases, load a new round, and cock the firing component after a round is fired. These include lever‐action, pump or slide‐action, bolt‐action, and semiautomatic long guns, the names of which refer to the loading mechanism used on each. Semiautomatic rifles use the force of the gas produced during firing to eject the spent cartridge case, load a new round, and cock the firing component. Semiautomatic fire‐arms use a disconnector mechanism to fire one shot per trigger pull, whereas fully automatic firearms do not have such a mechanism and fire multiple consecutive shots with a single pull of the trigger. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 6  7RENDWULQD6KXWWHUVWRFN & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 7  * HRUJH3LFNRZ 7KUHH/LRQV+ XOWRQ $ UFKLYH* HWW\,P DJHV & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 8  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 9  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG The broach cutter 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. 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. 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 firing componenting or heavy rollers into the mandrel’s form. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 10  5 LFKDUG6DIHUVWHLQ3K'  ,ODQ $ P LKDL3KRWR6WRFN,VUDHO$ ODP \ 6WRFN3KRWR & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 11  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG 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. 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 12 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. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 12  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 13  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG If one could cut a barrel open lengthwise, a careful examination of the interior would reveal the existence of fine lines, or striations, 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. No two rifled barrels, even those manufactured in succession, have identical striation markings. These striations form the individual characteristics of the barrel. As the bullet passes through the barrel, its surface is scratched by the rifling markings of the barrel. The bullet emerges from the barrel bearing the striations by the bore’s interior surface; these impressions reflect both the class and individual characteristics of the barrel. 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. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 14  /HLFD0 LFURV\VWHP V,QF & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG 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. 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. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 15  3HWHU' LDF]XN-RKQ-D\& ROOHJHRI& ULP LQDO-XVWLFH & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF $ OO5 LJKWV5 HVHUYHG 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. 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 if Colt does not manufacture any 16 firearms 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 microgrooving and may have 8 to 24 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. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 16  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG 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. choke An interior constriction placed at or near the muzzle end of a shotgun’s barrel to control shot dispersion. Choke is the degree of constriction placed at the muzzle end of the barrel. The greater the choke, the narrower the shotgun pattern and the faster and farther the pellets will travel. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 17  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG 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. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 18 19  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG The act of pulling a trigger releases the weapon’s firing pin, causing it to strike the primer, which in turn ignites the powder. The expanding gases generated by the burning gunpowder propel the bullet forward through the barrel, simultaneously pushing the spent cartridge case or shell back with equal force against the breechface. As the bullet is marked by its passage through the barrel, the shell is also impressed with markings by its contact with the metal surfaces of the weapon’s firing and loading mechanisms. As with bullets, these markings can be reproduced in test‐fired cartridges to provide distinctive points of comparison for individualizing a spent shell to a firearm. The shape of the firing pin is impressed into the relatively soft metal of the primer on the cartridge case, revealing the minute distortions of the firing pin. These imperfections may be sufficiently random to individualize the pin impression to a single weapon. Similarly, the cartridge case, in its rearward thrust, is impressed with the surface markings of the breechface. The breechface, like any machined surface, is populated with random striation markings that become a highly distinctive signature for individualizing its surface. Other distinctive markings that may appear on the shell as a result of metal‐to‐metal contact are caused by the extractor and ejector mechanism and the magazine or clip, as well as by imperfections on the fire chamber walls. Firing pin, breechface, extractor, and ejector marks may also be impressed onto the surface 20 of the brass portion of shells fired by a shotgun. These impressions provide points for individualizing the shell to a weapon that are just as valuable as cartridge cases discharged from a rifled firearm. Furthermore, in the absence of a suspect weapon, the size and shape of a firing pin impression and/or the position of ejector marks in relationship to extractor and other markings may provide some clue to the type or make of the weapon that may have fired the questioned shell, or at least may eliminate a large number of possibilities. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 20  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Search Systems The effort to build a national computerized database for firearms evidence in the United States had a rather confusing and inefficient start in the early 1990s. Two major federal law enforcement agencies, the FBI and the ATF, offered the law enforcement community competing and incompatible computerized systems. Early Systems The automated search system developed for the FBI was known as DRUGFIRE. This system emphasized the examination of unique markings on the cartridge casings expended by the weapon. The specimen was analyzed through a microscope attached to a video camera. The magnification allowed for a close‐up view to identify individual characteristics. The image was captured by a video camera, digitized, and stored in a database. Although DRUGFIRE emphasized cartridge‐case imagery, the images of highly characteristic bullet striations could also be stored in a like manner for comparisons. The Integrated Ballistic Identification System (IBIS), developed for the Bureau of Alcohol, Tobacco, Firearms and Explosives, processed digital microscopic images of identifying features found on both expended bullets and cartridge casings. IBIS incorporated two software programs: Bulletproof, a bullet‐analyzing module, and Brasscatcher, a cartridge‐case‐analyzing module. 21 NIBIN In 1999, members of the FBI and ATF joined forces to introduce the National Integrated Ballistics Information Network (NIBIN) program to the discipline of firearms examination. NIBIN guides and assists federal, state, and local laboratories interested in housing an automated search system. The new unified system incorporates both DRUGFIRE and IBIS technologies available in prior years. ATF has the overall responsibility for the system sites, whereas the FBI is responsible for the communications network. NIBIN serves only as a screening tool for firearms evidence. A computerized system does not replace the skills of the firearms examiner. NIBIN can screen hundreds of unsolved firearms cases and may narrow the possibilities to several firearms. However, the final comparison will be made by the forensic examiner through traditional microscopic methods. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 21  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG distance determination The process of determining the distance between the firearm and a target, usually based on the distribution of powder patterns or the spread of a shot pattern. Modern ammunition is propelled toward a target by the expanding gases created by the ignition of smokeless powder or nitrocellulose in a cartridge. Under ideal circumstances, all of the powder would be consumed in the process and converted into the rapidly expanding gases. However, in practice the powder is never totally burned. When a firearm is discharged, unburned and partially burned particles of gunpowder in addition to smoke are propelled out of the barrel along with the bullet toward the target. If the muzzle of the weapon is sufficiently close, these products are deposited onto the target. The distribution of gunpowder particles and other discharge residues around the bullet hole permits an assessment of the distance from which a handgun or rifle was fired. The accuracy of a distance determination varies according to the circumstances of the case. When the investigator is unable to recover a suspect weapon, the best that the examiner can do is to state whether a shot could have been fired within some distance interval from the target. More exact opinions are possible only when the examiner has the suspect weapon in hand and has knowledge of the type of ammunition used in the shooting. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 22  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 23  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 24 & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF $ OO5 LJKWV5 HVHUYHG 0 LUDQGD0 LFKHOOH'  Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 25 & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF $ OO5 LJKWV5 HVHUYHG 0 LUDQGD0 LFKHOOH'  Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 26 & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF $ OO5 LJKWV5 HVHUYHG 0 LUDQGD0 LFKHOOH'  Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 27 & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF $ OO5 LJKWV5 HVHUYHG 0 LUDQGD0 LFKHOOH'  Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 28  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG When garments or other evidence relevant to a shooting are received in the crime laboratory, the surfaces of all items are first examined microscopically for gunpowder residue. These particles may be identifiable by their characteristic colors, sizes, and shapes. However, the absence of visual indications does not preclude the possibility that gunpowder residue is present. Sometimes the lack of color contrast between the powder and garment or the presence of heavily encrusted deposits of blood can obscure the visual detection of gunpowder. Often, an infrared photograph of the suspect area overcomes the problem. Such a photograph may enhance the contrast, thus revealing vaporous lead and powder particles deposited around the hole. In other situations, this may not help, and the analyst must use chemical tests to detect gunpowder residues. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 29  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 30  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Nitrites are one type of chemical product that results from the incomplete combustion of smokeless (nitrocellulose) powder. One test method for locating powder residues involves transferring particles embedded on the target surface to chemically treated gelatin‐coated photographic paper. This procedure is known as the Greiss test. The examiner presses the photographic paper onto the target with a hot iron; once the nitrite particles are on the paper, they are made easily visible by chemical treatment. In addition, comparing the developed nitrite pattern to nitrite patterns obtained from test firings at known distances can be useful in determining the shooting distance from the target. A second chemical test is then performed to detect any trace of lead residue around the bullet hole. The questioned surface is sprayed with a solution of sodium rhodizonate, followed by a series of oversprays with acid solutions. This treatment causes lead particles to exhibit a pink color, followed by a blue‐violet color. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 31  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Efforts to identify a shooter now center on the detection of primer residues deposited on the hand of a shooter at the time of firing. With the exception of most.22‐caliber ammunition, primers currently manufactured contain a blend of lead styphnate, barium nitrate, and antimony sulfide. Residues from these materials are most likely to be deposited on the thumb web and the back of the firing hand of a shooter because these areas are closest to gases escaping along the side or back of the gun during discharge. In addition, individuals who handle a gun without firing it may have primer residues deposited on the palm of the hand coming in contact with the weapon. However, with the handling of a used firearm, the passage of time, and the resumption of normal activities following a shooting, gunshot residues from the back of the hand are frequently redistributed to other areas, including the palms. Therefore, it is not unusual to find higher levels of barium and antimony on the palms than on the backs of the hands of known shooters. Another possibility is the deposition of significant levels of barium and antimony on the hands of an individual who is near a firearm when it is discharged. SEM Testing Most laboratories possessing gunshot residue detection capabilities require the application of an adhesive to the shooter’s hands. Microscopic primer and gunpowder particles on the adhesive are then located with the aid of a scanning electron microscope (SEM). These 32 particles have a characteristic size and shape that readily distinguish them from other contaminants present on the hands. When the SEM is linked to an X‐ray analyzer, an elemental analysis of the particles can be conducted. A finding of a select combination of elements (lead, barium, and antimony) confirms that the particles could be primer residue. A complication to the interpretation of the significance of these findings is that brake linings and fireworks have been reported to yield particles indistinguishable from gunshot residue. Interestingly, bullet manufacturers are moving in the direction of removing lead from bullets, thus further complicating the characterization of primer residue. The major advantage of the SEM approach for primer residue detection is its enhanced specificity over hand swabbing. The SEM characterizes primer particles by their size and shape as well as by their chemical composition. Unfortunately, the excessive operator time required to search out and characterize gunshot residue has deterred the use of this technique. The availability of automated particle search and identification systems for use with scanning electron microscopes may overcome this problem. Results of work performed with automated systems show it to be significantly faster than a manual approach for searching out gunshot residue particles. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 32  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Determination of whether a person has fired or handled a weapon or has been near a discharged firearm is normally made by measuring the presence and possibly the amount of barium and antimony on the relevant portions of the suspect’s hands. A variety of materials and techniques are used for removing these residues. The most popular approach, and certainly the most convenient for the field investigator, requires the application of an adhesive tape or adhesive to the hand’s surface in order to remove any adhering residue particles. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 33  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Serial numbers are usually stamped on a metal body or frame, or on a plate, with hard steel dies. These dies strike the metal surface with a force that allows each digit to sink into the metal at a prescribed depth. Serial numbers can be restored because the metal crystals in the stamped zone are placed under a permanent strain that extends a short distance beneath the original numbers. When a suitable etching agent is applied, the strained area dissolves faster than the unaltered metal, thus revealing the etched pattern in the form of the original numbers. However, if the zone of strain has been removed, or if the area has been impressed with a different strain pattern, the number usually cannot be restored. Before any treatment with the etching reagent, the obliterated surface must be thoroughly cleaned of dirt and oil and polished to a mirrorlike finish. The reagent is swabbed onto the surface with a cotton ball. The choice of etching reagent depends on the type of metal surface being worked on. A solution of hydrochloric acid (120 mL), copper chloride (90 g), and water (100 mL) generally works well for steel surfaces. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 34  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG The Hollywood image of an investigator picking up a weapon by its barrel with a pencil or stick in order to protect fingerprints must be avoided. This practice only disturbs powder deposits, rust, or dirt lodged in the barrel, and consequently may alter the striation markings on test‐fired bullets. If recovery of latent fingerprints is a primary concern, hold the weapon by the edge of the trigger guard or by the checkered portion of the grip, which usually does not retain identifiable fingerprints. The most important consideration in handling a weapon is safety. Before any weapon is sent to the laboratory, all precautions must be taken to prevent an accidental discharge of a loaded weapon in transit. In most cases, it will be necessary to unload the weapon. If this is done, a record should first be made of the weapon’s firing component and safety position; likewise, the location of all fired and unfired ammunition in the weapon must be recorded. When a revolver is recovered, the chamber position in line with the barrel should be indicated by a scratch mark on the cylinder. Each chamber is designated with a number on a diagram, and as each cartridge or casing is removed, it should be marked to correspond to the numbered chambers in the diagram. Knowledge of the cylinder position of a cartridge casing may be useful for later determination of the sequence of events, particularly in shooting cases when more than one shot was fired. Place each round in a separate box or envelope. If the weapon is a pistol, the magazine must be removed and checked for prints and the chamber then emptied. 35 As with any other type of physical evidence recovered at a crime scene, firearms evidence must be marked for identification and a chain of custody must be established. Therefore, when a firearm is recovered, an identification tag should be attached to the trigger guard. The tag should be marked to show appropriate identifying data, including the weapon’s serial number, make, and model and the investigator’s initials. Place the unloaded firearm into a ridged box properly labeled for shipment to the examining forensic facility. When a weapon is recovered from an underwater location, no effort must be made to dry or clean it. Instead, the firearm should be transported to the laboratory in a receptacle containing enough of the same water necessary to keep it submerged. This procedure prevents rust from developing during transport. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 35  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Protection of class and individual markings on bullets and cartridge cases must be the primary concern of the field investigator. Thus, extreme caution is needed when removing a lodged bullet from a wall or other object. If the bullet’s surface is accidentally scratched during this operation, valuable striation markings could be obliterated. It is best to free bullets from their target by carefully breaking away the surrounding support material while avoiding direct contact with the projectile. Bullets, cartridge casings, and discharged shells from shotguns should just be placed in a container that is appropriately marked for identification. It is recommended that the investigator not directly mark these items with a scribe. In any case, the investigator must protect the bullet by wrapping it in tissue paper before placing it in a pillbox or an envelope for shipment to the crime laboratory. Minute traces of evidence such as paint and fibers may be adhering to the bullet; the investigator must take care to leave these trace materials intact. When semiautomatic or automatic weapons have been fired, the ejection pattern of the casings can help establish the relationship of the suspect to his or her victim. For this reason, the exact location of the place from which a shell casing was recovered is important information that must be noted by the investigator. 36 In incidents involving shotguns, any wads recovered are to be packaged and sent to the laboratory. An examination of the size and composition of the wad may reveal information about the type of ammunition used and the gauge of the shotgun. The clothing of a firearms victim must be carefully preserved so as to prevent damage or disruption to powder residues deposited around a bullet or shell hole. The cutting or tearing of clothing in the area of the holes must be avoided as the clothing is being removed. All wet clothing should be air‐dried out of direct sunlight and then folded carefully so as not to disrupt the area around the bullet hole. Each item should be placed in a separate paper bag. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 36  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Just as the machined surfaces of a firearm are impressed with random striations during its manufacture, the edges of a pry bar, chisel, screwdriver, knife, or cutting tool likewise display a series of microscopic irregularities that look like ridges and valleys. Such markings are left as a result of the machining processes used to cut and finish tools. The shape and pattern of such minute imperfections are further modified by damage and wear during the life of the tool. Considering the unending variety of patterns that the hills and valleys can assume, it is highly unlikely that any two tools will be identical. Hence, these minute imperfections impart individuality to each tool. If the edge of a tool is scraped against a softer surface, it may cut a series of striated lines that reflect that pattern of the tool’s edge. Markings left in this manner are compared in the laboratory through a comparison microscope with test tool marks made from the suspect tool. The result can be a positive comparison, and hence a definitive association of the tool with the evidence mark, when a sufficient quantity of striations match between the evidence and test markings. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 37  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 38  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG The tool or its impression may contain valuable trace evidence. Chips of paint adhering to the mark or tool provide perhaps the best example of how the transfer of trace physical evidence can occur as a result of using a tool to gain forcible entry into a building. Obviously, the presence of trace evidence greatly enhances the evidential value of the tool or its mark and requires special care in handling and packaging the evidence to avoid losing or destroying these items. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 39  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG The primary consideration in collecting impressions at the crime scene is the preservation of the impression or its reproduction for later examination in the crime laboratory. Before any impression is moved or otherwise handled, it must be photographed (a scale should be included in the picture) to show all the observable details of the impression. Several shots should be taken directly over the impression as well as at various angles around the impression. The skillful use of side lighting for illumination will help highlight many ridge details that might otherwise remain obscured. Photographs should also be taken to show the position of the questioned impression in relation to the overall crime scene. Although photography is an important first step in preserving an impression, it must be considered merely a backup procedure that is available to the examiner if the impression is damaged before reaching the crime laboratory. Naturally, it is preferable for the examiner to receive the original impression for comparison to the suspect shoe, tire, garment, and so forth. In most cases when the impression is on a readily recoverable item, such as glass, paper, or floor tile, little or no difficulty is presented in transporting the evidence intact to the laboratory. If an impression is encountered on a surface that cannot be submitted to the laboratory, the investigator may be able to preserve the print in a manner that is analogous to lifting a fingerprint. This is especially true of impressions made in light deposits of dust or dirt. A 40 lifting material large enough to lift the entire impression should be used. Carefully place the lifting material over the entire impression. Use a fingerprint roller to eliminate any air pockets before lifting the impression off the surface. A more exotic approach to lifting and preserving dust impressions involves the use of a portable electrostatic lifting device. The principle employed is similar to that of creating an electrostatic charge on a comb and using the comb to lift small pieces of tissue paper. A sheet of mylar film is placed on top of the dust mark, and the film is pressed against the impression with the aid of a roller. The high‐voltage electrode of the electrostatic unit is then placed in contact with the film while the unit’s earth electrodes are placed against a metal plate (earth plate). A charge difference develops between the mylar film and the surface below the dust mark so that the dust is attached to the lifting film. In this manner, dust prints on chairs, walls, floors, and the like, can be transferred to the mylar film. Floor surfaces up to 40 feet long can be covered with a mylar sheet and searched for dust impressions. The electrostatic lifting technique is particularly helpful in recovering barely visible dust prints on colored surfaces. Dust impressions can also be enhanced through chemical development. An aerosol product known as Snow Impression Wax is available for casting snow impressions. The recommended procedure is to spray three light coats of the wax at an interval of one to two minutes between layers, and then let it dry for 10 minutes. A viscous mixture of Class I dental stone is then poured into the wax‐coated impression. After the casting material has hardened, the cast can be removed. Several chemicals can be used to develop and enhance footwear impressions made with blood. In areas where a bloody footwear impression is very faint or where a subject has tracked through blood, leaving a trail of bloody impressions, chemical enhancement can visualize latent or nearly invisible footwear impressions. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 40  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 41  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 42  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF $ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 43  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF $ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 44  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG The evidential value of the impression is determined by the number of class and individual characteristics that the examiner finds. Agreement with respect to size, shape, or design may permit the conclusion that the impression could have been made by a particular shoe, tire, or garment, but one cannot entirely exclude other possible sources from having the same class characteristics. More significant is the existence of individual characteristics arising out of wear, cuts, gouges, or other damage. A sufficient number or the uniqueness of such points of comparison supports a finding that both the evidence and test impressions originated from only one source. When a tire tread impression is left at a crime scene, the laboratory can examine the design of the impression and possibly determine the style and/or manufacturer of the tire. This may be particularly helpful to investigators when a suspect tire has not yet been located. Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 45  & RS\ULJKW‹ 3HDUVRQ(GXFDWLRQ,QF$ OO5 LJKWV5 HVHUYHG Saferstein, Richard and Tiffany Roy. Criminalistics. (13th Edition). Pearson Education (US), 2020. 46