Loss of Kinetic Energy

The severity of a wound, as determined by the size of the temporary cavity, is directly related to the amount of kinetic energy lost in the tissue, not the total energy possessed by the bullet. If a bullet penetrates a body but does not exit, all the kinetic energy will be utilized in wound formation. On the other hand, if the bullet perforates the body and goes through it, only part of the kinetic energy is used in wound formation. Thus, bullet A with twice the kinetic energy of B may produce a wound less severe than B, because A

perforates the body whereas B does not. This, of course, assumes the bullets follow the identical paths through the body.

The amount of kinetic energy lost by a bullet depends on four main factors.1 The first is the amount of kinetic energy possessed by the bullet at the time of impact. This, as has been discussed, is dependent on the velocity and mass of the bullet.

The second factor is the angle of yaw of a bullet at the time of impact.1 The yaw of a bullet is defined as the deviation of the long axis of the bullet from its line of flight. When a bullet is fired down a rifled barrel, the rifling imparts a gyroscopic spin to the bullet. The purpose of the spin is to stabilize the bullet's flight through the air. Thus, as the bullet leaves the barrel, it is spinning on its long axis, which in turn corresponds to the line of flight. As soon as the bullet leaves the barrel, however, it begins to wobble or yaw. The amount or degree of yaw of a bullet depends on the physical characteristics of the bullet (its length, diameter, cross-sectional density), the rate of twist of the barrel, and the density of the air.

Angles of yaw have been determined with certainty only in military weapons. The maximum angle of yaw at the muzzle may vary from 1.5 degrees for a 150-gr. .30-06 Spitzer bullet, to 6 degrees for a 55-gr. .223 (5.56 mm X 45) bullet.8 Extremes in temperature can increase yaw and thus the stability of the bullet. Altering the rate of twist in the barrel or the weight of the bullet can also alter the angle of yaw. The AR-15/M-16, as originally designed, had a barrel twist of 1/14 in. (1/356 mm). This twist was too slow, however, so that bullets fired from the weapon were so unstable as to cause significant problems in accuracy. In order to correct this flaw and to stabilize the bullet, the twist rate was changed to 1/12 in (1/305 mm). While this twist rate was sufficient to stabilize the 55-gr. bullet, when the U.S. military adopted the 62-gr. bullet, this rifling was found to be too slow to stabilize the heavier bullet and the rifling was changed to 1/7 in. (1/178 mm)

The greater the angle of yaw of a bullet when it strikes the body, the greater the loss of kinetic energy.1 Because retardation of a bullet varies as the square of the angle of yaw, the more the bullet is retarded, the greater is the loss of kinetic energy.

As the bullet moves farther and farther from the muzzle, the maximum amplitude of the yaw (the degree of yaw) gradually decreases. At 70 yards, the degree of yaw for the 55-gr. .223 (5.56 X 45-mm) caliber bullet decreases to approximately 2 degrees.8 This stabilization of the bullet as the range increases explains the observation that close-up wounds are often more destructive than distant wounds. It also explains the observation that a rifle bullet penetrates deeper at 100 yards than at 10 feet.

Although the gyroscopic spin of the bullet along its axis is sufficient to stabilize the bullet in air, this spin is insufficient to stabilize the bullet when it enters the denser medium of tissue. Thus, as soon as the bullet enters the body, it will begin to wobble, i.e., its yaw increases.1 As the bullet begins to wobble, its cross-sectional area becomes larger, the drag force increases, and more kinetic energy is lost. If the path through the tissue is long enough, the wobbling will increase to such a degree that the bullet will become completely unstable, rotate 180 degrees and end up traveling base forward.

Tumbling of a bullet causes a much larger cross-sectional area of the bullet to be presented to the target. This in turn results in greater direct destruction of tissue as well as greater loss of kinetic energy and a larger temporary cavity. The sudden increaseof thedragforce ortumblingputs a great strain on the bullet which may eventually break up. A short projectile will usually tumble sooner than a longer one.

The third factor that influences theamountof kineticenergylostinthe body is the bullet itself: its caliber, construction, and configuration. Blunt-nose bullets, being less streamlined than spitzer (pointed) bullets, are retarded more by the tissue and thereforelose greateramountsof kinetic energy. Expanding bullets, which "open up" or "mushroom" in the tissue, are retarded more than streamlined fullmetal-jacketedbullets, which resist expansion and lose only a minimum amountof kineticenergyastheypass through thebody.

The caliber of a bullet and its shape, i.e., the bluntness of the nose, are important in that they determine theinitialvalueof theareaof interphase between the bullet and the tissue andthusthe "drag" of thebullet. Shape and caliber decrease in importance whendeformityof thebulletoccurs. The amount of deformation in turn depends on both the construction of the bullet (the presence or absence of thejacketing; thelength, thickness, and hardness of the jacket material; the hardnessof theleadusedinthebullet; the presence of a hollow-point) and the bullet velocity. Lead roundnose bullets will start to deform at a velocity above 340 m/sec (1116 ft/sec) in tissue. For hollow-points, it is above 215 m/sec (705 ft/sec).10

Soft-point and hollow-point centerfire rifle bullets not only tend to expand as they go through the body, but also shed lead fragments from the core (see Chapter 7, "Lead Snowstorm"). This shedding occurs whether or not they strike bone. The pieces of lead fly off the main bullet mass, acting as secondary missiles, contacting more and more tissue, increasing the size of the wound cavity and thus the severity of the wound. Such a phenomenon, the shedding of lead fragments, does not happen to any significant degree with handgun bullets, even if they are soft-point or hollow-point, unless they strike bone. Breaking up of missiles appears to be related to the velocity. The velocity of handgun bullets, even of the new high-velocity loadings, is insufficient to cause the shedding of lead fragments seen with rifle bullets.

Figure 3.3 X-ray of gelatin block struck by 55 gr. FMJ M-16 bullet illustrating breakup of bullet.

A fact not often appreciated is that full metal-jacketed rifle bullets may break up in the body without hitting bone. This phenomenon was not seen in the .30-06 (7.62 X 63 mm) M-1 round but gained considerable medical attention with the M-193, 55-gr., 5.56 X 45 mm, M-16 round (Figure 3.3). Thus, there were press and medical reports stating that this bullet "blows-up" in the body. The M-193 M-16 bullet does tend to break up after penetrating the body, but it does not blow up. Although this round has a reputation for causing extremely severe wounds, the amount of kinetic energy lost by this round is less than that from the relatively low-velocity .30-30 (circa 1895) hunting cartridge.

The tendency of a full metal-jacketed bullet to break up in the body is governed by its velocity and tendency to radically yaw.11 When the bullet yaws significantly, its projected cross- sectional area becomes much larger, with a resultant increase in the drag force acting on the bullet. The sudden increase in this drag force puts a great strain on the structure of the bullet, resulting in a tendency to break up. All this causes a greater loss of kinetic energy with an increase in the severity of the wound. Callendar and French, commenting on the tendency of high-velocity, full metal-jacketed bullets to break up, observed that blunt-nosed bullets break up from the tip, whereas pointed bullets break up from the base.2 In both types of full metal-jacketed bullets, the lead core can be squeezed out the base if the bullet is exposed to severe stress, due to tumbling.

Breakup of the military M 193, 55-gr., 5.56 X 45-mm bullet initiates when it begins to yaw. The bullet tends to flatten on its longitudinal axis and bend at the cannula. The tip of the bullet remains relatively intact while the core and rest of the jacket shred and lead is expelled out the base. Certain minimum velocities are necessary for this to occur. The bullets flatten at velocities in the low 2,000 ft/s range, breaking up in the mid to high 2,000 ft/s range.5 Breakup of the 62-gr. version of this bullet is similar. The 7.62 X 51 bullet also starts to break up at the cannula.11

The fourth characteristic that determines the amount of kinetic energy loss by a bullet is the density, strength, and elasticity of tissue struck by a bullet as well as the length of the wound track. The denser the tissue the bullet passes through, the greater the retardation and the greater the loss of kinetic energy. Increased density acts to increase the yaw as well as shorten the period of gyration. This increased angle of yaw and the shortened period of gyration lead to greater retardation and increased loss of kinetic energy.

One final point should be made about kinetic energy and temporary cavity formation. No matter how large a temporary cavity a bullet produces, it will have little or no effect unless it forms in an organ sensitive to injury from such a cavity. A 3-inch cavity in the liver is more effective as a wounding agent than the same cavity in the thigh muscle.

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