Multiple factors modify the ballistic properties of a given firearm. Shotgun ballistics is especially complex because multiple projectiles, called pellets and cumulatively termed “shot,” are fired from a single shotgun shell, versus a single projectile with each round for rifles and handguns. The standard shotgun has a nonrifled barrel that ranges from 18 to 32 inches (45.7-81.3 cm) long for the civilian market. The end of the barrel is threaded for the choke, a device that adds levels of constriction to the diameter of the barrel. The level of constriction and barrel length determine the diameter of the pattern of spread of pellets. More choke causes a narrower or tighter pattern to the fired pellets.1
The diameter of a shotgun barrel is differentiated by gauge, not caliber-like handguns and rifles. Gauge is calculated by the weight of a solid lead sphere that fits into the barrel bore. Gauge is the multiplicative inverse of a sphere’s weight in fractions of a pound. A 1/12th-pound lead sphere fits into a 12-gauge bore. The most popular shotgun gauges are the larger 12-gauge and smaller 20-gauge.2
Shotgun ammunition is most commonly termed as shells. Standard 12-gauge shells are typically sold in 2.75-inch (7.0-cm), 3-inch (7.6-cm), or 3.5-inch (8.9-cm) lengths. These differences in length allow for differing amounts of pellets and powder to be placed into each shell. A standard shotgun shell consists of a plastic casing with a metal base and primer, gunpowder, wadding to separate the pellets above from the powder below, and crimped ends to hold all of the components together. The wadding acts as a gas seal to maintain consistent pressures. There are many options on pellet size and composition. The size of pellets varies from large “000” buckshot, with each pellet measuring 0.36 inches (0.9 cm) in diameter, to small no. 9 birdshot, with pellets 0.08 inches (0.2 cm) in diameter. Pellets are typically made of lead, but may be steel. The typical 2.75-inch (7.0-cm) shell has 1 oz (28.3 g) of pellets. Smaller but more numerous pellets meeting this weight have less kinetic energy, whereas larger but fewer pellets have more kinetic energy.3,4
Shotgun shell pellets inherently do not have good ballistic qualities for distance, energy, and accuracy. Unlike rifled barrels that impart spin to a single conical projectile, a smoothbore shotgun barrel imparts no spin to spherical pellets that can impact each other to both deform the pellets and alter their trajectory. Greater numbers of smaller pellets have a better chance of hitting a closer target, but each pellet has a smaller amount of kinetic energy. Fewer, larger pellets have a longer range but are more likely to deviate from the point of aim and miss the target.5
Multiple variables with shotgun gauges and shotgun ammunition complicate shotgun ballistics. The spread of pellets upon leaving the muzzle determines the pattern size. At greater distances from the muzzle, the pellets spread farther from center of aim. The spread pattern, and even partial patterns, can be used to estimate the range of fire. There are major limitations to this estimation though, as the same shotgun and ammunition must be used for test fire for comparison, and only an estimation of ballistic properties can be derived.6
Because a shotgun shell has many pellets that are fired simultaneously, there is extensive interpellet interaction that allows for energy transfers that cause pellet energy transfer and dispersion. These energy transfers also impart slightly different ballistic properties to each pellet. This helps explain variable penetration of shotgun pellets into a target. Penetration depends on velocity, size/shape of pellet, and hardness of the pellet. Therefore, without knowing these exact variables, including the pellet’s position in the spread pattern, one cannot predict the probability of penetration for any single pellet.1,5
Firearm wound patterns for single projectiles (bullets) have been well studied. The pathophysiology of bullet-penetrating trauma depends on the range of fire as well as the path of the projectile through tissues, including a permanent and temporary cavity, which inflict damage by differing mechanisms of action.7 For shotgun wounds, the range of fire is the primary indicator of wound pattern.8
The purpose of this research was to determine the effect of common clothing fabrics as intermediate targets on the penetration of shotgun pellets at different ranges of fire. Ten percent ballistic gelatin has been widely accepted as an analog for soft tissue.9 When studying penetration versus no penetration, the tensile strength of skin may be significant. A skin stimulant was used in this study to ensure the most accurate tissue analog. Prior research provided the parameters of this skin stimulant.10
MATERIALS AND METHODS
A 12-gauge shotgun (Benelli Nova Pump, Benelli USA, Pocomoke, Md) with 28-inch (71.1-cm) barrel was used with a modified choke and no. 8 lead shot (Federal Ammunition, Federal Cartridge Company, Anoka, Minn, muzzle velocity 1145 ft/s [349 m/s]). The no. 8 pellets were 0.09 inches (0.2 cm) in diameter, and each shell contained 410 pellets.
Ballistics gelatin ( http://www.gelatininnovations.com) with a composition of 10% gelatin by weight was mixed with room temperature water and allowed to rest for 2 hours to allow for total absorption into the liquid. This mixture was then heated very slowly until fully melted. It was then placed into a refrigerator at 0°F (−17.8°C) for 12 hours. This process produced a consistent block with good clarity. Gelatin was cast into 9 × 9-inch (22.9 × 22.9-cm) and 13 × 9-inch (33.0 × 22.9-cm) blocks, each 2 inches (5.1 cm) in depth. The gelatin mixture was calibrated by firing a 0.177-caliber steel BB into the blocks. The blocks were wrapped in plastic and packed in a cooler with sealed ice packs until usage.9
A human skin simulant (SS) was used that consisted of thin, semifinished, chrome-tanned upholstery cowhide ( http://www.brettunsvillage.com). As a natural product, this cowhide has inherent variability, with a thickness ranging from 0.9 to 1.1 mm and weight of 18 oz/yd2 (600 g/m2) and tensile strength of 21 MPa. The tensile strength of normal human skin measured at the sternum is 19.4 MPa. In contrast, “bullet-resistant” Kevlar has a tensile strength of 3600 MPa.10
A total of 4 common clothing fabrics (Jo-Ann Fabrics and Craft Stores®, Inc, Hudson, Ohio) were used as intermediate targets. Fabrics vary by their weave or alignment of fibers (threads). A plain weave fabric has fibers aligned in warp and weft as a crisscross pattern. A denim fabric is stronger than plain weave because a weft fiber passes under 2 or more warp threads; weft threads are typically dyed white, and warp threads blue, giving a different appearance to each side of the fabric. A sweatshirt weave has fibers that are aligned loosely, giving the fabric a more bulky feel and greater insulating properties. Fabric weights are measured in ounces of fabric per square yard, or grams per square meter, and weight relates to the size of fibers and the density of the weave.11 Fabric weights, weaves, and fiber content for this study included 100% denim twill weave cotton, 10 oz/yd2 (330 g/m2); 100% sweatshirt weave cotton, 9 oz/yd2 (300 g/m2); 100% plain weave cotton, 6 oz/yd2 (200 g/m2); and 100% plain weave polyester, 3 oz/yd2 (100 g/m2).
Ballistics gelatin blocks (GBs) 9 × 9 × 2 inches (22.9 × 22.9 × 5.1 cm) were set at ranges of 40, 45, and 50 yd (36.6, 41.1, and 45.7 m) and 13 × 9 × 2 inches (33.0 × 22.9 × 5.1 cm) at 55 yd (50.3 m). The targets used at each of these ranges included GB only, GB + SS, GB + SS + denim, GB + SS + cotton, GB + SS + polyester, GB + SS + sweatshirt. The SS and fabric were held in place with a thin layer of low-density polyethylene wrap. Three sets of data were collected at each of the 4 yardages.
The data collected included the number of pellets that (1) hit the target, (2) penetrated into and stuck within the gelatin, and (3) went all the way through the gelatin. GraphPad software ( http://www.graphpad.com/quickcalcs/) was used to perform χ2 statistical analyses on the collected data showing significant (P< 0.05), highly significant (P < 0.001), and extremely significant (P < 0.0001) differences between the different targets for penetration into the gel and for penetration through the gel. GraphPad InStat software was used to perform a 1-way analysis of variance on these variables based on yardage.
Table 1 illustrates a percentage-based summary of the data. The percentage of pellets that hit a target and penetrated into the gel refers to those pellets that penetrated the intermediate target (skin + fabric) and embedded within the gel block. Figure 1 illustrates fabric with reduced penetration. The percentage of pellets that hit the target and went through the gel connotes pellets that penetrated completely through the gel block, represented as a cavity in the gel block.
χ2 Tests were performed on each variable to determine the significance between variables. The difference between skin and gel was insignificant at the closer ranges, with P values from 0.5521 to 1, but at 50 and 55 yd (45.7 and 50.3 m); the significance was evident by P values from 0.0173 to 0.0001. The fabrics + skin were also tested versus skin alone to determine the impact that the fabrics have on penetration and effective protection to the shotgun shell pellets. Denim + skin and cotton + skin were both significantly better than skin alone at all yardages, with P values ranging between 0.0261 and 0.0001. The sweatshirt + skin was also significantly better than skin alone except at the closest 40-yd (36.6-m) range. The polyester + skin added very little protection over skin alone and only at the most distant 50- and 55-yd (45.7- and 50.3-m) ranges.
There was no significant difference in penetration between fabrics including denim + skin and cotton + skin or sweatshirt + skin at any yardage. Cotton + skin and sweatshirt + skin also showed no statistical difference. At 40 to 50 yd (36.6–45.7 m), the polyester + skin had variable significance versus all other fabrics that ranged from a P value of 0.0022 to 1. At 55 yd (50.3m), the polyester + skin showed no difference in penetration compared with other fabrics.
The ballistic properties of shotgun projectiles are complex because of multiple variables of shot size, muzzle velocity, barrel length, barrel choke, pellet interaction, and range of fire.1–3 Despite these variables, we sought to determine characteristics for penetration of a target for a standard 12-gauge shotgun with a common commercially available shell with no. 8 lead pellets. We sought to assess the effects on target penetration with intermediate targets consisting of common clothing fabrics. We sought to simulate not only human soft tissue penetration with use of standard ballistics gelatin, but also human skin with use of thin tanned cowhide. We used GBs 2 inches (5.1 cm) in thickness to simulate a depth of soft tissue sufficient to reach vital organs such as heart, lung, liver, and large vessels. We set targets starting at 40 yd (36.6 m), because at closer ranges of fire, the tighter pattern of pellets made counting individual pellets difficult, and the energies retained produced penetration through the gelatin by more than half of pellets. We did not use targets at greater than 55 yd (50.3 m), because the wide dispersion of pellets at longer ranges meant few target strikes occurred, and it was more difficult to maintain gelatin consistency in larger cast blocks.
After statistical review of data (Table 1), several observations were made. At the closer ranges of 40 to 45 yd (36.6–41.1 m), the SS did not add any additional protection over the gelatin alone. At 50 and 55 yd (45.7 and 50.3 m), the SS did add protection. Because of the high coefficient of drag that round pellets have, their velocities and therefore kinetic energies diminish quickly at more distant ranges of fire. This explainsthe significance of using the SS when testing low-velocity projectiles such as shotgun shell pellets. The protection afforded by the SS is probably negligible with higher velocityprojectiles, such as handgun and rifle bullets.
Of the fabrics used as intermediate targets, the denim, cotton, and sweatshirt fabrics were all statistically indistinguishable from each other. All 3 significantly exhibited added protection over gelatin alone and SS at all ranges tested (Fig. 1). The polyester fabric, which was thinner with less weight, performed the worst of all the fabrics, but was slightly better than the SS alone at the more distant ranges of 50 and 55 yd (45.7 and 50.3 m). The polyester at shorter ranges displayed variable protection versus the other fabrics, as it was very inconsistent, sometimes being worse than the others while other times being just as protective against penetration.
At 55 yd (50.3 m) (Table 1), penetration was limited to no more than 1 pellet per gelatin target through any given fabric, whereas the SS alone exhibited a high penetration percentage. Also, at that distant range, there was no penetration that went the full 2 inches through the GB. Penetration to 2 inches (5.1 cm) may represent a fatal depth of penetration, whereas less than that could be considered more of a flesh wound.
Our data may be useful in interpreting firearms injury patterns involving shotguns in forensic cases. Clothing as an intermediate target was shown to modify the penetration patterns of shotgun shell pellets, at least at more distant ranges of fire. We observed variable penetration into targets simulating skin and soft tissues, confirming the complex ballistic properties of shotgun shell pellets. Our data show that yardage is an important factor in penetration. The data also indicate that the SS alone adds a great deal of protection, especially beyond ranges of 40 yd (36.6 m), against shotgun shell pellets. The fabrics also added protection over skin alone, as shown by Table 1.
In summary, our observations from this study showed that at the end velocities for the ranges of fire from 40 to 55 yd (36.6–50.3 m), the SS added protection against no. 8 lead shotgun pellet penetration, particularly at the higher ranges. Polyester fabric added little extra protection over the skin alone, but all other fabrics exhibited significant added protection. The range of fire is still a significant factor for possible pellet penetration. Not only is there more velocity and energy retained at shorter distances, but the pattern of the fired pellets is tighter, leading to more pellets hitting the target. There is a great deal of variability between the individual pellet ballistic properties as well the consistency of pattern density. We acknowledge that this was a focused study, but the experimental model can be used to study different shot sizes, different chokes, and different ranges of fire.
The authors thank the Georgia Department of Natural Resources Wildlife Resource Division for the use of their gun range.
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