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. A moving projectile, by virtue
of its movement, possesses kinetic energy. For a bullet, this energy is
determined by its weight and velocity:
Where g is gravitational acceleration,W is the weight
of the bullet, and V is the velocity. 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.
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. 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 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. 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. 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. 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. 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 × 45-mm) and
the .45–70 cartridges. The 5.56 × 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– 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. 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. 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. 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.” 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. 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. 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. 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. 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 greater the angle of yaw of a bullet
when it strikes the body, the greater the loss of kinetic energy, the more the bullet
is retarded, the greater is the loss of kinetic energy. 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. The sudden increase of the drag force or tumbling puts
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 the
amount of kinetic energy lost in the body is the bullet itself: its caliber,
construction, and configuration. Bluntnose bullets, being less streamlined than
spitzer (pointed) bullets, are retarded more by the tissue and therefore lose
greater amounts of kinetic energy. Expanding bullets, which “open up” or
“mushroom” in the tissue, are retarded more than streamlined full
metal-jacketed bullets, which resist expansion and lose only a minimum amount
of kinetic energy as they pass through the body. The amount of deformation in
turn depends on both the construction of the bullet (the presence or absence of
the jacketing; the length, thickness, and hardness of the jacket material; the
hardness of the lead used in the bullet; 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).
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. 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.
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. 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.
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.
Acknowledgements:
The Police Department;
www.politie.nl and a Chief Inspector – Mr. Erik
Akerboom ©
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