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 bullets 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.
    " 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 bullets 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.
    " 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.
    " It is the authors 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 authors 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.
    " 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 JFKs 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, JFKs head did move twicefirst 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 heada 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 JFKs brain, as would be required by a shot coming from the knoll, which was really to JFKs 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 JFKs head was hit by only one bullet, from the rear.

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