A missile crushes the tissue it strikes, thereby creating a permanent wound channel (permanent cavity). If the bullet is traveling with its pointed end forward and its long axis parallel to the longitudinal axis of flight (0° of yaw, the angle between the long axis of the bullet and its path of flight), it crushes a tube of tissue no greater than its approximate diameter. When the bullet yaws to 90°, the entire long axis of the bullet strikes tissue. The amount of tissue crushed may be three times greater than at 0° of yaw.
When striking soft tissue with sufficient velocity, soft-point and hollow-point bullets deform into a mushroom shape. This increases surface area and the amount of tissue crushed. For most big-game hunting, such bullets are mandated by law. This is to increase the probability of prompt lethality, rather than the creation of a disabling but nonlethal wound causing an animal prolonged suffering. If the mushroomed diameter is 2.5 times greater than the initial diameter of the bullet, the area of tissue crushed by the bullet is 6.25 times greater than the amount that would have been crushed by the undeformed bullet.
Bullet fragmentation also increases the volume of tissue crushed.8 After bullet fragmentation, bullet surface area is increased and much more tissue is crushed. For large handgun (e.g., .44 magnum) and rifle bullets, the striking of bone is one of the causes of early bullet fragmentation.
Comminuted fracture may be created by rifle and large handgun bullets striking bone. Bone fragments can become secondary missiles, crushing tissue. Many handgun bullets are unable to fragment bone significantly. When a large bone is struck, it is likely that the bullet will expend its wounding potential in the victim and will not exit.
Bullet fragments and secondary missiles, such as bone fragments, teeth, or coins, propelled by contact with the bullet are likely to increase the severity of the wound. Multiple perforations weaken tissue and create focal points for stress (stress risers). Tissue tears are particularly likely to occur at stress risers during temporary cavitation stretch.9
Unjacketed lead bullets cannot be driven faster than about 2000 ft/sec (610 m/s) without some of the lead stripping off in the barrel. This is avoided if a jacket made of a harder metal (such as copper or a copper alloy) is used to surround the lead. The jacket of a military bullet completely covers the bullet tip (a full metal jacket). Civilians often use hollow-point or soft-point bullets. Hollow-point bullets have a hole in the jacket at the bullet tip, and soft-point bullets have some of the lead core of the bullet exposed at the bullet tip. These constructions weaken the bullet tip, causing it to flatten on impact. This flattening often greatly exceeds bullet diameter, resulting in a mushroom-shaped projectile.
The hollow-point and soft-point bullets used by civilians are often more damaging to tissue than are military bullets fired from rounds otherwise configured identically.69 and 13 Because of this, wounds produced by civilian hunting rifles, shotguns, and large-caliber handguns are usually more severe than wounds produced by military-rifle bullets of the same mass and velocity.13
Hollow-point and soft-point bullets either deform into a mushroom shape or stay undeformed. Up to one-third of hollow-point and soft-point handgun bullets fail to deform into a mushroom shape, usually due to insufficient bullet velocity or an excessively stiff or thick bullet jacket that prevents deformation.
When the tip of a hollow-point bullet is plugged with material such as clothing or drywall, bullet expansion into a mushroom shape in tissue is usually delayed and sometimes prevented. This causes deeper penetration of tissue, sometimes causing a perforating wound (having both and entrance and an exit). This may result in the injury of bystanders. Some recent handgun bullets have designs attempting to overcome this problem.
Projectiles penetrate more deeply as projectile velocity is increased, but only up to the point where velocity becomes sufficiently high to deform the projectile. Penetration decreases markedly from that point on. The greater the bullet diameter expansion from mushrooming, the less is the depth of penetration. 14
There is a critical range of velocity for each handgun hollow-point and soft-point bullet, within which the bullet may perform as expected. Below this velocity range the bullet will have insufficient velocity to mushroom on impact, and at velocities above this range the bullet may fragment after impact, resulting in many light bullet pieces crushing tissue at a superficial depth.
Military full metal jacket bullets do not flatten at the bullet tip, i.e., they do not mushroom. Sometimes they can break and fragment as a result of yawing to 90°. The stress on the bullet as its long axis strikes tissue causes the sides of the bullet to flatten as if it had been squeezed in a vise. If the bullet breaks, it will usually do so at the cannelure, a circular groove around the bullet where it is crimped into the cartridge case. Although the M-193 military bullet of the M16 rifle fragments in soft tissue wounds with a characteristic pattern depending on range,15 most other full metal jacket military bullets, such as those fired from the AK-47, AK-74, and the NATO 7.62-mm rifle (US version), do not fragment unless they strike a large bone.
If a bullet is jacketed, the bullet jacket usually cannot be distinguished from the lead core on standard radiographs, because the entire bullet is metallic density. Occasionally, as the bullet deforms or fragments, the bullet jacket separates from the bullet and is visible on a radiograph.
In extremity wounds, when a radiograph reveals an undeformed bullet lying in the soft tissues and no fracture is present, tissue disruption is usually minor. If a major vessel or nerve is divided, however, even a simple wound can have a severe effect.
Wounding is like real estate: location is the most important factor. A bullet of low wounding potential can cause a severe wound if it passes through a vital structure such as the spinal cord.
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