Introduction
Firearms examiners are frequently called on to estimate the range from which a gunshot was fired. The distribution of gunshot residue on the target surface can be used for this determination. Gunshot residue consists of the following materials: (l)unburned propellant particles; (2)partially burned propellant particles; (3)soot from the combustion of propellant; (4)nitrates and nitrites from combustion of propellant; (5)particles of primer residue (oxides of lead, antimony and barium); and (6) particles of bullet or bullet jacket. The following terminology is frequently used to characterize ranges of fire based on the deposition of gunshot residue:
• Distant shot: the shot was fired from such a distance that no gunshot residue reached the target surface. Distant gunshot wounds are circular or elliptical defects surrounded by a marginal abrasion or contusion ring caused by the bullet stretching and tearing the skin. The contusion ring is frequently obscured by a gray ring of bullet ‘wipe’, consisting of lubricant and gunpowder combustion products. Distant bullet holes in clothing, walls and the like will lack the contusion ring but will usually show a ring of bullet ‘wipe’. An elemental analysis of the residue surrounding the hole may be required to determine whether a hole in an item of clothing, a door or a wall is in fact a bullet hole.
• Close-range shot: the shot was fired close enough to the target surface for some gunshot residue to reach it. For handguns close range typically means within 12-18 inches (30-45 cm). However, this is very much dependent on the type of ammunition being fired and the condition of the weapon. For rifles and shotguns close range typically means within several feet. The gunshot residue deposited on the target surface consists primarily of unburned propellant particles, partially burned propellant particles and soot. The unburned and partially burned propellant particles produce what is called ‘stippling’ or ‘tattooing’. The heated particles may be fused to the surface of the target. The particles may be driven into exposed skin (hence the name ‘tattooing’). As the distance between the gun muzzle and the target surface decreases the gunshot residue pattern becomes smaller and more concentrated, with increasing amounts of stippling and soot deposition. Gunshot residue patterns display a variety of patterns, such as irregular, circular or petal.
• Near-contact shot: the shot was fired at a range of 1 in (2.5 cm) or less. At this range, there will be a concentrated gunshot residue pattern. The muzzle flash (the incandescent gases issuing from the muzzle) will also interact with the target surface. Hair will be singed. Textile fibers may be singed or even melted and the textile structure will be disrupted. Woven textiles will split apart in an X-shaped pattern; knitted textiles will show a large circular or elliptical hole.
• Contact shot: the shot was fired with the muzzle in contact with the target surface. It is useful to distinguish loose contact shots from hard contact shots. A loose contact shot results when the muzzle of the firearm just touches the target surface. Gunshot residue may be blown out along the target surface or between layers of clothing. A hard contact shot results when the muzzle is pressed tightly against the target surface. Gunshot residue tends to follow the bullet into the target and not soil the target’s surface. The heat of the muzzle flash can scorch or melt textile fibers, producing a so-called ironing effect. Hard-contact gunshot wounds over bony plates (e.g. the vault of the skull) can produce characteristic stellate defects. These are produced when the hot propellant gases create a pocket between the overlying soft tissue and the bone (depositing gunshot residue on the surface of the bone). The soft tissue is pressed against the weapon’s muzzle, producing a patterned contusion. If the gas pressure is high enough the soft tissue can split open; blood and tissue may be blown back into the weapon’s muzzle. The soft tissue in loose and hard contact gunshot wounds frequently displays the cherry red color of carboxyhemoglobin, which is produced by the reaction of hemoglobin with carbon monoxide in the propellant gases.
In order to conduct a range-of-fire determination the firearms examiner requires certain pieces of evidence or information. He should have the original gunshot residue pattern, the weapon believed to have fired the pattern, ammunition from the same lot as that used to fire the original gunshot residue pattern and knowledge of the weather conditions prevailing at the time of the shooting. In the case of a gunshot residue pattern on skin a scaled black and white or color photograph of the pattern is usually used for comparison purposes. However, black and white photographs may be misleading as some of the discoloration of the skin in a powder pattern is the result of a vital reaction of the skin to the impact of hot propellant grains. Even a color photograph may be misleading. Skin blemishes may be mistaken for stippling or tattooing. Ideally, the firearms examiner should be present at the autopsy of a shooting victim so that he can examine the gunshot residue pattern himself. Older homicide investigation texts frequently recommended excising the skin bearing gunshot residue patterns at autopsy and sewing the formalin-fixed tissue to a metal hoop. The advisability of this practice is open to question: the formalin-fixed tissue will shrink to some degree; the evidence may be deemed by a judge too inflammatory to be admitted at trial; and members of some religions would regard the excision of the powder pattern as an unconscionable mutilation of the dead.
Gunshot residue patterns on clothing may not be readily visible, particularly if they are on a dark fabric or obscured by blood. In such a case several techniques can be used to visualize the gunshot residue pattern. In the case of patterns obscured by blood, infrared photography can render the pattern visible; blood is virtually transparent to infrared radiation, whereas soot is a strong infrared absorber. Infrared imaging devices have also been successfully used to visualize gunshot residue patterns. Chemical tests may also be used to visualize a gunshot residue pattern on a dark-colored garment or one that is bloodstained. C acid (2-naphthylamine-4,8-disulfonic acid) reacts with traces of nitrates and nitrates in gunshot residue patterns to produce dark red spots. In the Griess test nitrites in the gunshot residue pattern react with the Griess reagent (sulfanilic acid and 2-naphthylamine or sulfanilamide and N-(1-naphthy-l)ethylenediamine) to produce an azo dye by means of the familiar diazotization reaction. For the C acid and Griess tests the gunshot residue pattern is transferred to a piece of desensitized photographic paper that has been impregnated with the appropriate reagent. The garment is placed on a towel on a laboratory benchtop with the powder pattern uppermost; the impregnated photographic paper is placed with the gelatin-coated side down and covered with a towel. A hot iron is then pressed over the entire pattern to transfer the powder pattern to the photographic paper.
The sodium rhodizonate test is also used in range-of-fire determinations. Sodium rhodizonate reacts with lead primer residue to produce a blue lead rhodizonate complex. This blue complex can be converted into a scarlet complex by treatment with tartrate buffer (pH 2.8). For the sodium rhodizonate test the pattern is transferred to a sheet of filter paper, which is then sprayed with the sodium rhodizonate reagent, followed by the tartrate buffer solution. Alternatively, the tartrate buffer solution may be saturated with sodium rhodizonate and this saturated solution used to develop the gunshot residue pattern. The sodium rhodizonate test may be made specific for lead by spraying the scarlet pattern produced by the sodium rhodizonate/tartrate buffer combination with dilute hydrochloric acid to produce a blue-violet pattern. Barium ion produces a red complex with sodium rhodizonate; however, the color of this complex is not affected by changing the pH. Mercurous ion, ferrous ion and thiosulfate ion give red or orange colors with sodium rhodizonate solution but these colors fade when treated with dilute hydrochloric acid. Ammunition containing lead-free primers will of course not produce residue containing lead and the sodium rho-dizonate test will not produce a color reaction with powder patterns fired with such ammunition. If the ammunition primer contains zinc peroxide zincon can be used to develop a primer residue pattern.
Once the firearms examiner has visualized the powder pattern, he will test-fire the suspect weapon into cloth targets set at different distance from the muzzle of the weapon until a gunshot residue pattern is obtained that closely approximates the questioned pattern in size and density. Care must be taken to use ammunition from the same lot as that believed to have been used to fire the questioned powder pattern. For this reason, police investigators should collect as evidence any unfired ammunition in the possession of the suspect. Failure on the part of defense experts in the John Donald Merrett case to use the same weapon and ammunition from the same lot as that used to produce the questioned powder pattern led to an erroneous opinion as to the range from which the shot that killed Merrett’s mother was fired; Merrett was acquitted and went on to commit two other murders. Stippling and soot deposition on the test-fired patterns is readily apparent to the naked eye; however, primer residue patterns on the test-fired targets must be visualized by chemical treatment, using sodium rhodizonate or zincon, as appropriate.
The range from which a shotgun pellet pattern was fired can also be estimated. Although the rule of thumb that a shotgun pellet pattern will spread one inch for every yard the shot mass travels down range may be used for rough estimates, careful range-of-fire determinations require that the firearms examiner have the original pellet pattern, the shotgun believed to have fired it and shotshells from the same lot as that used to fire the questioned pellet pattern. The examiner also needs information about the weather conditions: wind speed and direction obviously effect the size and density of the pellet pattern; ambient temperature also effects the size of the pellet pattern (presumably by changing the rate of combustion of the propellant in the shotshell). The firearms examiner will test fire pellet patterns into paper or cloth targets from a number of different ranges. The examiner may visually compare the test-fired pellet patterns with the questioned pattern and continue his test-firings until he obtains a test-fired pellet pattern that appears to match the questioned pattern in overall size and density. This simple approach can yield surprisingly accurate results. In a blind study ten pellet patterns were fired at randomly selected ranges between 6 feet and 41 feet (2-12.5 m). Visual comparison of these ten patterns with test-fired patterns resulted in range-of-fire estimates with an average absolute error of only 1.5 ft (45 cm) and an average relative error of 6.8%.
Various attempts have been made to introduce statistical methods such as regression analysis into the estimation of the range of fire from a pellet pattern. Statistical methods require some measure of the size of a pellet pattern. If DH is the horizontal spread of the pellet pattern, Dv the corresponding vertical spread and N the number of pellets, the following measures of the size of a shotgun pellet pattern can be defined:
If xi and ji are the Cartesian coordinates of the ith pellet hole in an arbitrary coordinate system then:
Finally, if R is the radius of the smallest circle that will just enclose the pellet pattern, R can be measured using a clear plastic overlay marked off with a series of concentric circles. Studies have shown that DH, Dv, < D >, D, S and R are generally linear functions of the range of fire, whereas A and d are not. The calculation of S is laborious and has little to recommend it for practical work.
Regression analysis is familiar to most scientists and a variety of statistical computer software packages are currently available which permit the calculation of regression equations and more importantly the confidence limits for the estimated ranges of fire. The major problem with the application of regression analysis to the problem of range of fire estimation is the large number of test-fired patterns required to determine the regression equation for pellet pattern size versus range of fire. A minimum of 20 test-fired patterns would be required for useful results. A firearms examiner rarely has available this many shot-shells of the same lot as that used to fire the questioned pellet pattern. Two alternative approaches have been proposed for the determination of the confidence limits of estimated ranges of fire. In the first, three or more shots are test fired at each of several ranges and the largest and smallest pellet pattern sizes are plotted graphically. The lower confidence limit is the range at which the size of the questioned pellet pattern corresponds to the size of the largest pellet pattern size; the higher confidence limit is the range at which the size of the questioned pellet pattern corresponds to the size of the smallest pellet pattern size. This procedure requires no assumptions about how the shotgun pellets are distributed in the pellet pattern. On the other hand, it is virtually impossible to determine the confidence level of the confidence limits. Confidence limits based on an assumed normal distribution of pellets within the pattern have also been explored. However, experimental evidence indicates that the distribution of pellets within a pattern is unlikely to be Gaussian. For example, 00 buckshot patterns typically consist of a series of superimposed triangles (for a nine-pellet load, two nearly coincident triangles with the third rotated roughly 60° with respect to the first two). No. 2 shot patterns have been shown to have bimodal distributions.
An additional complication in the estimation of the range of fire from a shotgun pellet pattern is the phenomenon called the ‘billiard ball effect’. This term refers to the spreading of a shotgun pellet pattern caused by an intermediate target. When shotgun pellets exit a shotgun barrel they initially travel as an elongated mass. If the pellet mass encounters an intermediate target the leading pellets will be slowed down; the trailing pellets will overtake the leading pellets, colliding with them and thus causing them to fly off at eccentric angles. If the pellets go through the intermediate target to strike the final target, the resulting pellet pattern may be larger than would be produced at the same muzzle-to-final-target distance in the absence of the intermediate target. In other words, the pellet pattern in the final target may appear to have been fired from a greater distance than was actually the case. The larger (and hence more massive) the pellets the denser the intermediate target must be in order for it to produce a billiard ball effect. Metal or plastic window screen will produce a significant billiard ball effect with no. 2 birdshot, but the effect of these materials on 00 buckshot is trivial. Human skin can act as an intermediate target; consequently, care should be taken in making estimates of ranges of fire from radiographic images. As the muzzle-to-intermediate-target distance increases the billiard ball effect diminishes and eventually disappears altogether. As the pellets travel away from the muzzle the pellets in the shot mass separate from one another; eventually each pellet is traveling on its own trajectory and the slowing of the leading pellets by the intermediate target will not result in collisions between pellets. The firearms examiner may be alerted to the existence of an intermediate target by the presence of markings (e.g. from window screen) or trace evidence (e.g. paint or wood splinters) on the shotgun pellets.
