Bullets disrupt tissues by two principal mechanisms:
- Laceration and crushing which are generated by the bullet displacing the tissues in its track (“drilling effect”) and are recognized as the primary wounding mechanism produced by low velocity (< 1000 feet per second) gunshot wounds e.g. from handguns. The degree and amount of laceration and crushing are dependent upon missile velocity, shape, angle of impact, yaw, and tumbling, though shape and construction of a bullet are not significant factors at such low-velocities as observed in handguns. In such cases the permanent cavity seen at autopsy accurately reflects the tissue damage produced by the path of the bullet.
- Cavitation: The wounding effect of the cavitation phenomenon is only significant in velocities surpassing 1,000 feet per second. When a missile enters the body, the kinetic energy imparted on the surrounding tissues forces them forward and radially in the wake of the missile, causing the wound cavity to be stretched outward, creating a “temporary” cavity or temporary displacement of tissues in the projectile’s path. Produced by the large amount of kinetic energy transferred to the tissue, this cavity may be up to 30 times the diameter of the projectile, has a lifetime of 5 to 10 milli-seconds, produces pressures of 100 to 200 atmospheres and collapses into the “permanent” cavity or wound (bullet) track in a pulsatile fashion. The “permanent” cavity is caused by the path of the bullet itself, the diameter of which is variable but usually larger than the diameter of the bullet. If the pressure of temporary cavity formation exceeds the elastic limit of the tissue, then the organ may be disrupted (“bursts”) and a large permanent cavity seen reflects the size of the temporary cavity. Organs which are dense, (and thus cause greater loss of projectile KE), and relatively inelastic are most susceptible to this bursting effect e.g. liver. Organs with low density and high elasticity are relatively protected e.g. lung. Organs such as muscle and skin, which have similar density to liver, are relatively protected because of their elasticity. High-velocity gunshot wounds of the head produce bursting injuries of the skull due to temporary cavity formation. A missile’s ability to produce a temporary cavity is considered an important component in wound production and degree of destruction especially in high-velocity rifle wounds, e.g. centrefire hunting rifles
Other wounding mechanisms include:
- Shockwaves receding from the bullet at the speed of sound, they cause compression of tissues that lay ahead of the bullet, as well as to the sides, but these waves last only a few microseconds and do not cause profound destruction at low velocity, thus are not a major factor in most handgun wounds. At high velocity, generally, > 2,500 feet per second, generated shock waves can reach up to 200 atmospheres of pressure. These may cause the rupture of gas-filled organs such as the bowel.
- Secondary projectiles such as shattered bone fragments e.g. gunshot wounds to the head.
- Discharge gases exiting the muzzle of the weapon pass into the wound track in hard contact wounds, and produce tissue disruption e.g. contact shotgun wounds to the head.