SHOTGUN WOUND PATTERNS

Transcription

1 TIIK AMERICAN JOURNAL OF CLINICAL PATHOLOGY Copyright 1969 by The Williams & Wilkins Co. Vol. 52, No. 3 Printed in U.S.A. SHOTGUN WOUND PATTERNS RUDIGER BREITENECKER, M.D. Department of Pathology, Greater Baltimore Medical Center, Baltimore, Maryland ABSTRACT Breitenecker, Rudiger: Shotgun wound patterns. Am. J. Clin. Path., 52: 25S-269, Relatively little has been published in the world literature on shotgun wound interpretation and ballistics. Since only few medical centers see a large number of shotgun injuries, less experienced examiners may be misled easily hi their interpretation of such wounds, particularly with regard to estimates of distance, with grave implications as to judicial consequences. The peculiarities of the shotgun and the shot shell frequently permit more accurate range estimates than do hand weapons, especially for ranges beyond 1 ft. Choke is the most important variable factor influencing the shot scatter pattern in the average medicolegal case, whereas gauge has little effect on pattern size. The most accurate estimation of range is achieved by test firing the same gun and ammunition at varying ranges to compare the spread with the entrance wound. The wound characteristics largely depend upon the distance. Close range shots make a single punched out entrance hole that increases in size proportionally to the range until individual pellets enter the target. Concomitantly, the internal injuries vary, ranging from complete destruction of the target organ at close range to individual pellet wounds at greater distances. However, it is important to realize that pellet scatter patterns within the body may be similar in diameter in both close range and distant shots. Because of this similarity, estimation of range based upon body roentgenograms has been found to be less reliable than was believed previously and, indeed, may be very misleading. Exit wounds are very rare, even at contact range, and occur only hi tangential shots, when thinner portions of the body are injured, or with the use of large pellets (buckshot). One prime question that the medical examiner is asked in the evaluation of shotgun wounds is that of the distance between victim and gun. If the distance determined places the gun beyond the reach of the victim, a self-inflicted wound is obviously improbable, except under the unusual circumstance when the gun is triggered by a mechanical device. Thus, suicide usually can be ruled out and the shooting must be attributed to another person. Estimation of distance in the usual medicolegal case rarely involves ranges beyond 10 yards. The average human target is relatively small and greater distances usually cannot be determined accurately, since the spread of pellets may have exceeded the size of the target. Conclusions can only be drawn if both the center of the pattern and its periphery are available for examination. 9 Received May 17, GUN AND AMMUNITION Because of the nature of shotgun missiles the wounds are considerably different from other gunshot wounds. As with other firearms, the pellets are not the only objects discharged from the muzzle by the force of the powder explosion. In addition to different materials such as gas, powder (burned, partially burned, or unburned), and materials from the barrel (pieces of metal, rust, dirt, lubricant, etc.), the shotgun blast includes a variety of wadding (cardboard, felt or plastics, or both, and a variety of pellets 8 (Fig. 1). Pellets may vary in size from very fine shot, such as No. 12 (1.27 mm. in diameter), to the most commonly used load, No. 6 (2.79 mm. in diameter), to a single ball or rifled slug with a diameter nearly equal to that of the lumen of the barrel. Their number depends on the size of the shot and the gauge

2 Sept SHOTGUN WOUND PATTERNS 259 */-PO*t* (A) (B) (C) Fig. 1(A). Standard shotgun shell with overshot wad (1), paper tube (#), undershot, filler, and overpowder wads (S), powder (4). (B) and (C). Examples of a modern shell. Folded closing crimp replaces overshot wad (/), shot enclosed in plastic shot protector (#), plastic tube (S), shell on right with combination of plastic wad and felt wads (4), shell on left with plastic wad only (4), smokeless powder (5), metal head (6), base wad (7), primer (8). (Courtesy of Federal Cartridge Corporation.) of the gun. For instance, No. 12 shot contains 2385 pellets and No. 6 shot contains 223 pellets per ounce (Table l). 6 The diameter of the wads is slightly larger, about 0.01 in., than the bore itself. Despite the complexities of shotgun ballistics, the very nature of a shot shell for a shotgun frequently permits more accurate range estimates than are possible when a rifled weapon is used. A very significant factor influencing the scatter pattern in shotgun wounds is the type of weapon used. Of all the variables such as gauge (size), barrel length, and degree of choke, the latter is the most important. Choke is a constriction of the bore of the muzzle, usually confined to the distal 1 in. of the barrel, although occasionally it may be a long tapering type, extending as far back as 12 in. The total amount of constriction in a choke may vary from to in. This slight variation has profound effects on the diameter of the spread pattern. In the 5- to 20-yard range the spread diameter of a true cylinder (no choke) may be twice that of a full choke. Double-barreled guns frequently have various combinations of cylinder or choke bores. It should not be forgotten that some weapons may have interchangeable barrels or that a variable choke may be fitted to the barrel. The ap-

3 2G0 BREITENECKER Vol. 52 TABLE 1 AMERICAN SHOT SIZES Chilled Shot Drop (Soft) Shot Sliot No. Diameter Weight per pellet Approximate no. of shot/ounce Weight per pellet Approximate no. of shot/ounce Dust IVI B Air rifle BB BBB T TT F FF mm in Gm it Gm S' Buckshot Eastern size Western size Diameter in Approximate no./lb. Weight in mm in Gm «' proximate spread of shot in inches at various distances is listed in Table It is, therefore, necessary to know the type of barrel used before accuracy in estimating distance can be expected. The degree of choke for all gauges is regulated by the manufacturer by shooting at a distance of 40 yards and adjusting the barrel so that a desired percentage of shot falls within a 30-in. circle. 5,6 The percentage of shot falling within a 30-in. circle at various choke adjustments is listed in Table 3. Barrel length is of somewhat less concern because in the great majority of medicolegal cases, standard, usually cheap weapons with 28-in. barrels are employed. Also, there is no apparent difference in pattern size due to barrel length from 20 in. to 36 in. 12 Of course, it is well recognized that the shorter the barrel (such as in sawed-off shotguns) the

4 Sept SHOTGUN WOUND PATTERNS 261 TABLE 2 APPROXIMATE SPREAD OF SHOT IN INCHES AT VARIOUS DISTANCES TABLE 3 PERCENTAGE OF SHOT FALLING WITHIN A 30-IN. CIRCLE AT VARIOUS CHOKE ADJUSTMENTS 5 yards 10 yards 15 yards 20 yards Choke Adjustment Average Cylinder Half-choke Full-choke wider the spread diameter at any given range. This is the main reason weapons with very short barrels lose their effectiveness and accuracy at any but close range. Gauge refers to the diameter of the barrel. The most commonly used size in this country is the 12-gauge bore. Diameters of shotgun barrels of various common gauges are: 10 gauge in. 12 gauge in. 16 gauge in. 20 gauge in. 28 gauge in gauge in. For practical purposes, the gauge of the shotgun is the least important to scatter pattern interpretation. One of the most common misconceptions is that a larger bore (gauge) produces a larger pattern than a smaller sized weapon and vice versa. In fact, for practical purposes, the diameter of spread of all gauges is about the same, since it is not the gauge, but mainly the degree of choke, which governs the diameter of the scatter pattern. 5 ' 9 However, the density of the pattern is influenced by the gauge. A larger sized shell contains more pellets which will strike at a given distance than a smaller shell. Also, the smaller the pellet, the greater the number contained in a given shell. This explains the maxim that "the smaller the shot, the denser the pattern," even if the total spread diameter is nearly the same. "WOUND PATTERNS It is understandable from the above considerations that accurate estimates of the range can only be obtained if the character- Full choke Three-quarter choke (improved modified) Half-choke Quarter-choke (modified choke) Improved cylinder True cylinder (no choke) % istics of both gun and shell are known to the examiner; however, some general principles apply to most standard weapons. GENERAL CHARACTERISTICS OF ENTRANCE WOUNDS ("PRIMARY TARGET" SKIN) Close range. At contact or near contact up to about 12 in. there will be a single, round wound, about % to 1 in. in diameter. The wound will show the usual characteristics of an entrance wound, a single inshoot hole surrounded by a narrow abrasion margin, sometimes with an innermost circle of dirt deposits from the passage of the missile. Smoke and powder deposits surrounding the entrance wound produce extensive blackening, even with modern "smokeless" powder, forming a circle up to about 6 in. in diameter. At near contact range these deposits frequently form two concentric circles (Fig. 2). Short range. Similar characteristics are seen in distances up to about 3 ft., except that the circle of blackening decreases in size. Slight blackening and tattooing (from burned or partially burned powder particles) are present with distances up to about 3 or even 4 ft., which is considerably different from rifled hand weapons, from which such deposits are rarely seen beyond 12 in. Intermediate range. At about 4 ft. the single entrance hole measures up to about \ A /i to 2 in. in diameter and its margin shows scalloping (Fig. 3). At about 6 ft. there are signs of outside pellets being separated from the main bulk of the charge, producing few individual shot wounds in the periphery of the main hole.

5 262 BREITENECKER Vol. 52 found within the body cavities or organs. A typical wad injury of the skin adjacent to the center of the charge can be found when the main wad has separated from the pellets, at a range of about 3 to 6 yards. This usually consists of a somewhat round excoriation, rarely with an actual penetration of the skin. Such injuries are seldom produced at greater distances, notwithstanding the fact that under ordinary conditions the main wad may be projected for 20 or even 40 yards. If a shot charge passes through a primary target such as glass, wood, or screen, before secondarily striking the body surface, the scatter pattern is much wider than might be expected and estimation of distance from the entrance wound on the skin becomes extremely hazardous. Only if correlated with a scene investigation or test firings can such estimates be meaningful. This phenomenon may even be seen when the charge of a very close range shot passes through heavy clothing. The shot dispersion may then produce an entrance wound in the underlying skin considerably larger than in the garment, resembling the size of a shot of a distance of several feet. This is why it is mandatory that F I G. 2(top). E n t r a n c e wound at 1 in. (2.5 cm.) muzzle-to-skin distance. Wound diameter %in. (2.2 cm.); 12-gauge, improved cylinder bore; No. 6 shot. F I G. 3(bottom). E n t r a n c e wound at 48 in. (120 cm.) distance. Wound diameter 2 in. (5 cm.). No fouling or tattooing. Note scalloped edges, b u t no individual pellet wounds; 12-gauge, improved cylinder bore; No. 6 shot. Distant range. Beyond the range of about 10 to 12 ft. there is great variation of spread, depending on gun and ammunition. However, in general, the separation of the whole charge into individual pellets is almost complete at this range (Fig. 4). A "rule of thumb" states that between 3 and 10 yards the dispersion of shot is about 1 in. per yard for a fully choked barrel and between 13^ and 2 in. per yard for a cylindrical barrel.11 I t is important to avoid a false sense of accuracy with such approximations and to remember the individuality of the weapon under consideration. As long as the shot charge enters the body through a single inshoot, the wads will be F I G. 4. E n t r a n c e wound at 16 ft. (about 5 m.) distance. Note two wad impacts, one with partial penetration of the skin. Spread diameter 9 to 10 in. (22 to 25 cm.); 12-gauge, improved cylinder bore; No. 6 shot.

6 Sept SHOTGUN WOUND PATTERNS 263 FIG. 5. Contact entrance hole in heavy jacket with disproportionately larger skin entrance wound (secondary target), due to shot dispersion by primary target (jacket); 12-gauge, No. 6 shot (shell with plastic cup). Suicide. FIG. 6. Chest organs from case shown in Figure 2. Massive destruction of heart and lung; 1-in. (2.5 cm.) muzzle-to-skin distance.

7 264 Vol. 52 BREITENECKEK body and clothing be examined in shooting victims (Fig. 5). INTERNAL INJURIES Close range wounds are extremely destructive and may produce near disintegration of many organs, disproportionately larger than the relatively small entrance wound in the skin. This is mainly due to the explosive forces of the powder gases, together with the wide dispersion of shot which occurs in the "intermediate target" (the various organs) after the charge has passed through the "primary target" (skin or clothing) 2 ' 10 (Fig. 6). These destructive forces are particularly evident in close, range wounds of the head, where shattering of skull and expulsion of brain are commonly seen (Fig. 7). Actual destruction of organs decreases as the cluster of pellets entering the body through a common opening becomes smaller. At greater range, when the pellets have become dispersed, individual shot wounds of the organs are the rule (Figs. S and 9). EXIT WOUNDS Exit wounds after penetration of the full thickness of the trunk generally are not seen, even at contact range. Usually all the energy of the pellets has been expended during their passage through the organs, and their velocity upon reaching the opposite skin is too low to allow penetration. Exceptions are large pellets such as 00 buckshot, tangential shots without passage through the full thickness of chest or abdomen, and injuries to a thinner portion of the body such as the neck or extremities. If there is an exit wound, it will be large and lacerated if the entire load i I FIG. 7. Contact wound in right posterior parietal region. Scattering of skull with complete expulsion of brain and destruction of cerebellum. Suicide. Trigger pushed with wooden stick; 12-gauge. Blood alcohol 0.15%.

8 Sept SHOTGUN WOUND PATTERNS 265 of shot emerges. If individual pellets stray from the main cluster and retain enough force of penetration after passage through a thinner part of the body, the exit wound is small and somewhat punctate (Figs. 10 and 11). PRESERVATION AND INTERPRETATION OV EVIDENCE It is important to search the wound for both shot and wads. Measurement of pellet size and weight will allow identification of the type of pellets used. Blood-soaked wads are often hidden among blood clots aid tissue fragments, requiring diligent search to recover them. After rinsing and drying, the wad may be measured as an aid in determining the gauge of the weapon. However, the examiner must use extreme care, because the wad tends to swell in blood and body fluids and may lose its usual round shape. 13 It should be remembered that the wads are normally slightly larger than the bore of the barrel. More recently, the multiple individual wads have been replaced by a single wad of plastic construction which is much easier to recover. The most accurate estimation of range is achieved by testfiring the same gun and ammunition at varying ranges to compare the spread pattern with the entrance wound. Accurate measurements of all wound characteristics are prerequisite to a reliable estimate. One should not forget that a single test shot may not necessarily give the full and true answer, since there may be significant variations even ia the same lot of ammunition. FIG. S. Chest organs from case shown in Figure 3. Note decrease of tissue destruction of heart and, particularly, lung, as compared with Figure 6; 48-in. distance (120 cm.).

9 266 BREITENECKER Vol. 52 FIG. 9. Chest organs from case shown in Figure 4. Note individual pellet wounds in heart, pericardial sac and lungs without massive tissue destruction. 16 feet distance (about 5 m.).

10 Sept SHOTGUN WOUND PATTERNS 267 FIG. 10. Tangential entrance wound in chest and gaping exit wound in left axilla. Note eccentric margin of inshoot and few satellite pellets in periphery of exit wound. Suicide; 12-gauge, No. 6shot. Blood alcohol 0.26%. FIG. 11. Homicidal shooting at about 16 to 20 ft. with two shots. Tangential perforation of left arm from above toward below. Note small exit wounds and few retained subcutaneous pellets in ulnar region. Separate abdominal entrance wound; 20-gauge, full choke, No. 4 shot; with plastic cup. (Courtesy Dr. W. U. Spitz, Baltimore, Md.)

11 268 BREITENECKER Vol. 52 FIG. 12. Excised skin entrance wound, compared with test patterns. Same gun and ammunition used in test firings permits estimate of range between 5 and 8 ft.; 20-gauge, No. 6 shot. Homicide. Another method, in addition to simply measuring the entrance wound, is to excise and fix the portion of skin involved and compare the actual and test patterns directly. One word of caution: it is essential to outline and accurately measure the margins of excision and of the main entrance hole first, since skin elasticity will cause the specimen to shrink immediately after excision; fixation magnifies this effect. Only if the specimen is mounted on a hard surface and pinned down to the actual dimensions before fixation can a meaningful result be expected (Fig. 12). As with bullet wounds, the contact ring of entrance or powder stains and tattooing may be obscured on dark-colored cloth. In such instances, infrared photography may be valuable in detecting and accentuating the black specks of powder or soot against the surface of the cloth which appears light colored with infrared film. Recently, L. Breitenecker suggested the use of infrared image converters in such cases. 1 On occasion, roentgenographic examination is used to demonstrate the location of pellets in the body. In the past, this procedure sometimes has been used to aid in range estimation when, for various reasons such as destruction by fire, animal feeding, or decomposition, the external wound was not available for examination. However, it has recently been shown that such practice can be hazardous and misleading, because both near contact wounds and wounds of several yards distance may give similar scatter pat-

12 Sept SHOTGUN WOUND PATTERNS 269 terns within the body. Serious mistakes may occur if a wide scatter pattern of a close shot within the body prompts the investigator to deduce a range greater than possible for a self-inflicted wound. In examining such partially destroyed bodies, it is, therefore, better to rely on adequate interpretation of the organ damage found at autopsy than to depend upon potentially misleading interpretation of roentgenographic findings alone. 34 REFERENCES 1. Breitenecker, L.: Personal communication. 2. Breitenecker, R.: Experimental studies of shotgun patterns in the human body. In Proceedings of the Seventh Congress of the International Academy for Legal and Social Medicine, Budapest, 19C7. 3. Breitenecker, R., and Senior, W.: Shotgun patterns, I. An experimental study on the influence of intermediate targets. J. Foren. Sci., 12: , Breitenecker, R.: Shotgun patterns, II. J. Foren. Sci., in press. 5. Burrard, G.: The Modern Shotgun, Vol. Ill, Ed. 2. London: Ltd. 1948, Herbert Jenkins, Drake, V.: Shotgun ballistics 1 and 2. J. Foren, Sci. Soc. (Harrow gate), 2: 85-93, 1962; S: 22-32, Gonzales, T. A., Vance, M., Helpern, M., and Umberger, C. J.: Legal Medicine, Pathology and Toxicology, Ed. 2. New York: Appleton- Century-Ciofts, Inc., 1954, pp Guerin, P. F.: Shotgun wounds. J. Foren. Sci., 5: , Hatcher, J. S., Jury, F. J., and Weller, J.: Firearms Investigation, Identification and Evidence. Harrisburg, Pa.: Stackpole, 1957, pp Janssen, W., and Kistner, D.: Injuries caused by a new type bird shot ammunition fired at close range. Arch. Kriminol., HI: 3-13, Moritz, A. R.: The Pathology of Trauma. Philadelphia: Lea & Febiger, 1942, p Munhall, B. D.: Personal communication. 13. Snyder, L. M.: Homicide Investigation, Ed. 2. Springfield, 111.: Charles C Thomas, Publisher, 1967, pp