Wound Ballistics

Ballistics is the science of the motion of projectiles. It is divided into interior ballistics, external ballistics, and terminal ballistics. Interior ballistics is the study of the projectile in the gun; exterior ballistics, the study of the projectile through air; and terminal ballistics, the study of penetration of solids by the missile. Wound ballistics can be considered a subdivision of terminal ballistics concerned with the motions and effects of the projectile in tissue. In this chapter we shall review wound ballistics.

A moving projectile, by virtue of its movement, possesses kinetic energy. For a bullet, this energy is determined by its weight and velocity:

K.E. = WV 2/2 g where g is gravitational acceleration, Wis the weight of the bullet, and Vis the velocity.1

From this formula, it can be seen that velocity plays a greater role in determining the amount of kinetic energy possessed by a bullet than does weight. Doubling the weight doubles the kinetic energy, but doubling the velocity quadruples the kinetic energy.

The concept of a gunshot wound held by most individuals is that of a bullet going through a person like a drill bit through wood, "drilling" a neat hole through structures that it passes through. This picture is erroneous. As a bullet moves through the body, it crushes and shreds the tissue in its path, while at the same time flinging outward (radially) the surrounding tissue from the path of the bullet, producing a temporary cavity considerably larger than the diameter of the bullet.1,2 This temporary cavity, which has a lifetime of 5 to 10 msec from initial rapid growth until collapse, undergoes a series of gradually smaller pulsations and contractions before it finally disappears, leaving the permanent wound track (Figure 3.1). It is the combination of the crushed and shredded tissue and the effects of the temporary cavity on tissue

Figure 3.1 Temporary cavity produced in gelatin block by 110-gr. semi-jacketed hollow-point .38 Special bullet.

adjacent to the bullet path (shearing, compression, and stretching) that determines the final extent of a wound.

The location, size, and the shape of the temporary cavity in a body depend on the amount of kinetic energy lost by the bullet in its path through the tissue, how rapidly the energy is lost, and the elasticity and cohesiveness of the tissue. The maximum volume and diameter of this cavity are many times the volume and diameter of the bullet. Maximum expansion of the cavity does not occur until some time after the bullet has passed through the target. The temporary cavity phenomenon is significant because it has the potential of being one of the most important factors in determining the extent of wounding in an individual. For this potential to be realized, however, not only must a large temporary cavity be created but it must develop in strategically important tissue, e.g., a cavity in the liver is more significant than one located in the thigh.

In the case of handgun bullets, the bullet produces a direct path of destruction with very little lateral extension within the surrounding tissues, i.e., only a small temporary cavity is produced. As a general rule, the temporary cavity plays little or no role in the extent of wounding. To cause significant injuries to a structure, a handgun bullet must strike that structure directly. The amount of kinetic energy lost in the tissue by the bullet is insufficient to cause the remote injuries produced by a high-velocity rifle bullet.

The picture is radically different in the case of a high-velocity rifle bullet. As the bullet enters the body, there is a "tail splash," or backward hurling of injured tissue. This material may be ejected from the entrance. The bullet passes through the target, creating a large temporary cavity whose maximum diameter is up to 11 to 12.5 times the diameter of the projectile.3 The maximum diameter of the cavity occurs at the point at which the maximum rate of loss of kinetic energy occurs. This occurs at the point where the bullet is at maximum yaw, i.e., turned sideways (at a 90° angle to the path) and/or when it fragments. If fragmentation does not occur and the path is long enough, the yawing continues until the bullet rotates 180° and ends up in a base-forward position. The bullet will continue traveling base first with little or no yaw as this position puts the center of mass forward.

The temporary cavity will undulate for 5 to 10 msec before coming to rest as a permanent track. Positive and negative pressures alternate in the wound track, with resultant sucking of foreign material and bacteria into the track from both entrance and exit. In high-velocity centerfire rifle wounds, the expanding walls of the temporary cavity are capable of doing severe damage. There is compression, stretching and shearing of the displaced tissue. Injuries to blood vessels, nerves, or organs not struck by the bullet, and a distance from the path, can occur as can fractures of bones, though, in the case of fractures, this is relatively rare.3 In the author's experience, fractures usually occur when the bullet perforates an intercostal space fracturing ribs above and below the bullet path.

The size of both the temporary and the permanent cavities is determined not only by the amount of kinetic energy deposited in the tissue but also by the density and elastic cohesiveness of the tissue. Because liver and muscle have similar densities (1.01 to 1.02 and 1.02 to 1.04), both tissues absorb the same amount of kinetic energy per centimeter of tissue traversed by a bullet.4 Muscle, however, has an elastic, cohesive structure; the liver, a weak, less cohesive structure. Thus, both the temporary and the permanent cavities produced in the liver are larger than those in the muscle. In muscle, except for the bullet path, the tissue displaced by the temporary cavity returns to its original position. Only a small rim of cellular destruction surrounds the permanent track. In liver struck by high-velocity bullets, however, the undulation of the temporary cavity loosens the hepatocytes from the cellular supporting tissue and produces a permanent cavity approximately the size of the temporary cavity. Lung, with a very low density (specific gravity of 0.4 to 0.5) and high degree of elasticity, is relatively resistant to the effects of temporary cavity formation, and has only a very small temporary cavity formed with very little tissue destruction.4

It is not the high velocity of the rifle bullet per se that is responsible for the aforementioned picture, but rather the amount of kinetic energy possessed by the bullet by virtue of the high velocity and which is deposited in the tissue. With most modern rifles, the kinetic energy possessed by the bullet is acquired by virtue of high velocity. A high level of kinetic energy can also be acquired by increasing the mass of the bullet, though this is not as efficient. To illustrate this point, consider the .223 (5.56 X 45-mm) and the .45-70 cartridges. The 5.56 X 45-mm cartridge, fired in the M-16 rifle series, is the most famous of the new high-velocity military cartridges. It fires a 55-gr. bullet at 3250 ft/sec with a muzzle kinetic energy of 1320 ft-lbs (1790 J). The .45-70 U.S. government black powder cartridge, adopted by the U.S. Army in 1873, fired an all-lead bullet of 405 gr. at a velocity of 1285 ft/s and with a muzzle kinetic energy of 1490 ft-lbs (2020 J), 170 ft-lbs (230.5 J) more than that of the .223 bullet. These bullets, a light-weight, high-velocity one and a heavy, slow-moving one, possess relatively equivalent amounts of kinetic energy and, thus, are capable of producing identical-sized temporary cavities. What will determine their effectiveness is where in the body they will produce their respective cavities.

Energy loss along a wound track is not uniform. Variations may be due either to behavior of the bullet or changes in the density of the tissue as the bullet goes from one organ to another. An increase in kinetic energy loss is reflected by an increase in the diameter of the temporary cavity. A full metal-jacketed rifle bullet will produce a cylindrical cavity until it begins to yaw. At this time, the bullet's cross-sectional area will become larger, and the drag force will be increased. The result is an increase in kinetic energy loss and thus an increase in the diameter of the temporary cavity (Figure 3.2A). In addition to the increase in size of the temporary cavity, there will also be an increase in the amount of tissue crushed as the bullet is presenting a larger impacting surface area. For the 7.62-mm NATO M 80 bullet, gelatin studies reveal that yawing begins after 15 cm of penetration, with maximum tissue disruption at approximately 28 cm where the yaw is 90 degrees.3

Projectile fragmentation can amplify the effects of the temporary cavity increasing the severity of a wound (Figure 3.3). This is the reason for the effectiveness of the 5.56 x 45-mm cartridge and the M-16 rifle. For the M-193 55-gr. bullet, on the average, the yaw becomes significant at 12 cm with marked tissue disruption occurring most commonly at 15 to 25 cm due principally to bullet fragmentation.3,5

In contrast to full metal-jacketed military bullets, with hunting ammunition, the bullet begins to expand (mushroom) shortly after entering the body, with a resultant rapid loss of kinetic energy. Thus, a large temporary cavity is formed almost immediately on entering the body (Figure 3.2B). This is augmented by shredding of the lead core.

A lead shotgun pellet produces a cone-shaped temporary cavity with the base of the cone at the entrance (Figure 3.2C). The diameter of the cavity gradually lessens as the velocity of the pellet decreases. The loss of velocity is much more rapid for shotgun pellets because of their unfavorable ballistic properties (large cross-sectional area in relation to mass).

It has been found that above a certain critical velocity 800 to 900 m/sec (2625 to 2953 ft/sec), the character of a wound changes radically with tissue destruction becoming much more severe.2 Trans- or supersonic flow within

Figure 3.2 Appearance of temporary cavities in gelatin blocks due to (A) full metal-jacketed rifle bullet, (B) hunting rifle bullet, and (C) shotgun pellet.

the tissue causing strong shockwaves has been assumed to be responsible for this effect. In experiments by Rybeck and Janzon, 6-mm steel balls weighing 0.86 g were fired at the hind legs of dogs.6 They found that at a velocity of 510 m/sec, the volume of macroscopically injured muscle was only slightly larger than the diameter of the bullet. At 978 and 1313 m/sec, the volume of devitalized muscle was seen to be 20 to 30 times the volume of the permanent cavity.

It is the author's belief that rather than there being a critical velocity above which the severity of wounds increases dramatically, there is instead a critical level (amount) of kinetic energy loss above which tissue destruction becomes radically more severe. This level is different for each organ or tissue. When a bullet or missile exceeds this kinetic energy threshold, it produces a temporary cavity that the organ or tissue can no longer contain, i.e., one that exceeds the elastic limit of the organ. When the elastic limit is exceeded, the organ "bursts." For full metal-jacketed rifle bullets and steel balls to reach this critical level of kinetic energy loss, these missiles must be traveling at

Figure 3.2 Appearance of temporary cavities in gelatin blocks due to (A) full metal-jacketed rifle bullet, (B) hunting rifle bullet, and (C) shotgun pellet.

very high velocities (greater than 800 to 900 m/sec; 2625 to 2950 ft/s). For soft-point and hollow-point rifle bullets, however, the same loss of kinetic energy will occur at lower velocities as a result of the deformation and breakup of the bullets. Thus, in the author's experience, for hunting bullets the critical velocity, appears to be between 1500 and 2000 ft/sec (457 to 610 m/sec).

In the case of hunting ammunition for centerfire rifles, no matter the caliber, once the critical level of kinetic energy lost in an organ is reached, the extent of destruction is relatively the same. Thus, these wounds generally do not appear any different in severity, regardless of the caliber of the rifle.

Centerfire rifle wounds of the head are especially destructive because of the formation of a temporary cavity within the cranial cavity. The brain is enclosed by the skull, a closed, rigid structure that can relieve pressure only by "bursting." Thus, high-velocity missile wounds of the head tend to produce bursting injuries. That these bursting injuries are the result of temporary cavity formation can be demonstrated by shooting through empty skulls. A high-velocity bullet fired through an empty skull produces small entrance and exit holes with no fractures. The same missile fired through a skull containing brain causes extensive fracturing and bursting injuries.7 Wounds due to hunting bullets are more destructive to the structure of the head than wounds produced by military ammunition even if the same weapon is used. This is because, even though both bullets may possess the same amount of energy on impact, the hunting bullet will lose more energy in the head due to its construction.

With a centerfire rifle bullet, the permanent cavity in tissue is usually larger in diameter than the bullet. With a low-energy projectile such as a handgun bullet, the permanent track is often distinctly smaller in diameter. Tissue elasticity with contraction of the surrounding tissue accounts for this latter phenomenon. If, however, the elastic limit of the tissue has been exceeded by the handgun bullet, the tissue tears, and a large irregular wound track is produced. This latter phenomenon is seen most often in the liver.

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