How
a high-speed bullet damages an organ
Spring 2000
High-speed bullets damage organs in ways different from what we usually
think. Here is an extended passage from one of the U.S.’s foremost authorities
on the subject, Dr. Vincent J. M. Di Maio, Chief Medical Examiner and Director
of the Regional Crime Laboratory, County of Bexar, San Antonio, Texas (from his Gunshot
Wounds, CRC Press, Boca Raton, FL, 1985).
"In the concept of a gunshot wound held by most
individuals, the bullet goes through the person like a drill bit through wood, 'drilling'
a neat hole through structures it passes through. However, this concept is
erroneous. As a bullet moves through the body, it imparts kinetic energy to the
surrounding tissue, flinging it away form the bullet’s path in a radial manner
(direction) and producing a temporary cavity considerably larger than the
diameter of the bullet. 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 [Figures 3-1 and 3-2].
"The size and the shape of the temporary cavity 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. [Emphasis
added.] The temporary cavity phenomenon is significant because it has been
found to be the most important factor in determining the extent of the wounding
in an individual in regard to the interaction of a bullet with the body. In the
case of low-velocity missiles, e.g., pistol bullets, the bullet produces a
direct path of destruction with very little lateral extension within the
surrounding tissues. Only a small temporary cavity is produced. To cause
significant injuries to a structure, a pistol bullet must strike that structure
directly. The amount of kinetic energy lost in the tissue by a pistol bullet is
insignificant to cause the remote injuries produced by a high-velocity rifle
bullet.
"The picture is
radically different in the case of a high-velocity missile. As the bullet enters
the body, there is a "tail splash," or the backward hurling of injured
tissue. The bullet passes through the target, creating a large temporary cavity
whose maximum diameter may be up to 30 times the diameter of the original
bullet. The maximum diameter of the cavity occurs at the point at which the
maximum rate of loss of kinetic energy occurs. This cavity will undulate for 5
to 10 msec before coming to rest as a permanent track. In high-velocity
centerfire rifles, the expanding walls of the temporary cavity are capable of
doing severe damage. Local pressures on the order of 100 to 200 atm may develop.
This pressure may produce injuries to blood vessels, nerves, or organs that are
a considerable distance from the path of the bullet. Fractures can occur even
without direct contact between the bone and a rifle bullet. Positive and
negative pressures alternate in the wound, with resultant sucking of foreign
material and bacteria into the wound from both entrance and exit.
"¼ 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 in an increase
in the diameter of the temporary cavity. A full metal-jacketed rifle bullet will
produce a cylindrical cavity until it begins to tumble. 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. With hunting ammunition, the picture is
radically different. The bullet will begin to expand shortly after entering the
body, with a resultant rapid loss of kinetic energy. A large temporary cavity is
formed immediately as the bullet enters the body.
"¼ It has been found that above
a certain critical velocity (800 to 900 m/sec or 2625 to 2953 ft/sec), the
character of a wound changes radically with tissue destruction becoming much
more severe. Trans- or super-sonic flow within 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 gm were fired at the hind legs
of dogs. 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.
"¼ 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 of kinetic
energy loss. This level is different for each organ or tissue. When a bullet
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 bullets or steel balls to reach that level of kinetic
energy and thus a particular size of 
temporary
cavity, these missiles must be traveling at very high velocities (greater than
800 to 900 m/sec). For soft-point or hollow-point rifle bullets, however, the
same loss of kinetic energy will occur at lower velocity as a result of the
deformation of the bullets. Thus for hunting ammunition, the critical velocity,
in the author’s experience, appears to be between 1500 and 2000 ft/sec (457.2
to 609.6 m/sec).
"High-velocity missile wounds of the head are especially destructive
because of 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.
"¼ 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 s the bullet enters to body, it will begin to wobble. 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
be completely unstable and will tumble end-over-end through the tissue."
Implications for the physics of JFK’s head shot
Note how the bullet interacts with the head in two stages. In the first
stage, the bullet passes rapidly through, leaving an expanding temporary cavity
in its wake. The separate events of this first stage are (1) the bullet enters
the skull by drilling a small entrance hole; (2) some brain matter is ejected
backward out this hole (tail splash); (3) the bullet, beginning to tumble,
passes through the brain; and (4) the bullet leaves the skull by blasting a
large exit hole. Note how each of these four events transfers forward momentum
from the bullet to the head (the first movement of the head).
In the second stage (after the bullet has left the skull), brain matter
continues moving radially outward from the path of the bullet until the head
bursts from the accumulated pressure, which can reach 100 to 200 atmospheres.
Brain matter is ejected out all available openings, the largest of which will
usually be the exit wound or an expanded version of it, with its size depending
on how large the internal pressures became. As brain matter is ejected through
the enlarged exit wound, it exerts a recoil force in the opposite direction, or
backward. If this force is strong enough relative to other forces being
experienced by the head at that time (which is well after the hit), the head may
actually move backward (the so-called "jet effect"). If the recoil
force is small relative to those other forces (such as neuromuscular reactions),
the head may more in some other direction, with its motion being only modified
by the jet effect.
Thus we expect a bursting head to show at least two separate movements. The
first must be in the direction of the bullet, the second probably opposite to
it. (Specifics of any movements beyond the first are difficult or impossible to
predict, however.) In fact, JFK’s head did move twice—first briefly forward
(the "snap"), then backward (the "lurch"). The quick
forward motion proves that the killing shot came from the rear. The rearward
motion was likely some combination of jet effect and a neuromuscular stiffening
of the back muscles, which together straightened him up and threw him backward.
Could the rearward lurch have been the result of a second bullet, from the
front, as implied in JFK? No, for several reasons: (1) There was only one
set of wounds to the head, a tiny entrance wound in the rear and a larger exit
wound on the right side/rear. (2) There is only one pattern of lead fragments in
the head—a cone fanning out from the rear entrance wound to the side exit
wound. (3) There was no second diffuse cloud of tissue and large fragments, as
created by the first hit. (4) There was no damage to the left hemisphere of
JFK’s brain, as would be required by a shot coming from the knoll, which was
really to JFK’s right rather than to his front. (5) The rearward lurch was an
entirely different kind of movement from the forward snap. The lurch began in
the right shoulder and arm and involved the head only later. It did not look at
all like the snap. (6) Because the lurch involved the whole upper torso, it
required more energy than the snap. Many weapons did not have enough energy.
Thus JFK’s head was hit by only one bullet, from the rear.