Ballisticsthe wound

The quest for the perfect bullet by law enforcement agencies and bullet designers centers on the concept of 'stopping power'. This concept is a fallacy because no bullet is capable of total incapacitation in and of itself. Aim and the organ struck will determine the degree to which an assailant is incapacitated. Only a bullet striking the brain or the high cervical spinal cord will prevent an assailant from any further action. Hemorrhage to the point of cerebral hypoxia and hypoperfusion will render incapacitation when 25 per cent or more of the blood volume is lost. A male of average size, able to compensate fully for progressive hypotension, could maintain adequate mentation for at least 4.5 s, even with a freely bleeding transected aorta. This is sufficient time for the entire magazine of a semi-automatic weapon to be emptied before the assailant is neutralized (Newgard 1992).

The mechanics by which bullets inflict injury is frequently ascribed to the transfer of kinetic energy from the bullet to the tissue. In contrast with stored or potential energy, kinetic energy (KE) is the energy of an object in motion, and is given by the formula

KE=wA2

where m is the mass of the object and v is its velocity. Conceptually, the larger and faster the bullet, the more damage it causes. However, energy transfer is a key concept in the kinetic energy description of terminal ballistics. Theoretically, a faster lighter bullet could do more damage than a slower heavier bullet because it has a higher velocity and therefore more kinetic energy (Table 1). However, the smaller bullet may actually travel through tissue without significant energy transfer and thus will be less effective in imparting damage.

Alteration in bullet design can profoundly change the amount of energy transferred. For example, a non-deforming bullet (e.g. full metal jacket) is more likely to travel easily through tissue, while a bullet that expands and deforms (e.g. hollow-point or soft-point) will be slowed during its travel. The difference between entry and exit velocity will determine the amount of energy transferred to the tissues (T.ib.!ยง 2). Similarly, as a higher-velocity missile encounters tissue and is slowed by its yaw, more kinetic energy will be transferred to the tissue, theoretically inflicting more damage.

Table 2 Energy transfer of a 9 mm bullet with full metal jacket compared with that of a jacketed hollow-point

The results of studies of the actual physical effects of projectiles using ordnance gelatin, which closely simulates human skeletal muscle ( Post..and..Johnson..1995), and high-speed videography correlate with injury patterns seen clinically. As the bullet travels through tissue, a temporary cavity (a pathway which collapses) and a permanent cavity are created.

CD Figure 1. As the bullet passes through tissue, temporary and permanent cavities are formed. Note the enlarged permanent cavity as the bullet deforms.

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CD Figure 2. Standard bullet fired from a .22 caliber rifle. Permanent and temporary cavities expand as the bullet tumbles, eventually with the heavier trailing edge forward.

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CD Figure 3. This bullet, fired from the US military M-16, is virtually the same diameter as that used in the .22 long rifle and somewhat heavier, but has a velocity nearly three times as fast. This marked increase in velocity leads to fracturing of the bullet, with fragments sent throughout the tissue. The effects of temporary cavitation are much greater as a result of the multiply lacerated tissue. Note that although these effects are severe, they do not begin until the bullet has traversed approximately 10 cm of tissue. Therefore the severe tissue damage associated with this bullet may not be seen if the thickness of tissue impacted is less than 10 cm (e.g. an extremity).

CD Figure 4. The 7.62 mm NATO bullet is one of the most common military rifle rounds in the world. Alhough it travels at almost 915 m/s, its usual steel core prevents it from fracturing, so that injury is limited to the immediate crush of the bullet's trajectory and temporary cavitation. Note the relatively long "neck" (15 cm) before cavitation begins.

CD Figure 5. Very different tissue destruction occurs when the jacketed steel core bullet is changed to a soft-point. Bullet fragmentation is almost immediate, with widespread destruction and extensive cavitation.

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