Dr. Vincent DiMaio, a renowned expert in wound ballistics, says that the 5.56 x 45 mm cartridge fired from the M-16 rifle series (including the AR-15) is the most famous of the available high-velocity cartridges.
This round is a 55 (or 62) grain bullet that leaves the rifle muzzle at 3250 feet per second (fps) – roughly three times the speed of modern handgun rounds.
DiMaio explains that when a full metal jacketed 5.56/.223 round contacts human tissue it will travel along a circular path while beginning to yaw or turn sideways. This turning effect will become significant at 12 cm (approximately 4.7 inches).
At the point of maximum yaw, the bullet will be turned at a 90-degree angle as it moves forward in the body.
If the bullet remains intact, it will yaw to 180 degrees and continue to travel base first until it comes to rest.
Handgun rounds do not yaw while traveling through human tissue.
This turning or yaw effect releases tremendous kinetic energy into the tissue surrounding the permanent cavity created by direct bullet contact with human tissue. Not only is the permanent cavity larger due to the sideways path of the bullet, thereby destroying more tissue through direct bullet contact, but the surrounding tissue, i.e., tissue not directly touched by the bullet, is severely impacted as well. This surrounding tissue is called the temporary wound cavity.
DiMaio reports that this temporary cavity will have a diameter from 11 to 12.5 times the diameter of the bullet itself.
Damage to the tissue in the temporary cavity will include “severe…compression, stretching, and shearing of the displaced tissue. Injuries to blood vessels, nerves, or organs not struck by the bullet, and at a distance from the [bullet] path can occur.”
Even bones not directly struck by the bullet itself, although rare, can be fractured by the kinetic energy emanating from the bullet’s contact with body tissue. Handgun rounds do little or no damage to the tissue, blood vessels, arteries, organs or bones located outside the permanent cavity. Temporary cavity damage from handgun wounds is a non-factor in handgun wound damage.
DiMaio also comments on another factor not present with handgun rounds, which involves the likelihood of high-velocity rifle rounds breaking up or fragmenting while passing through human tissue. DiMaio states that “[p]rojectile fragmentation can amplify the effects of [damage to] the temporary cavity increasing the severity of a wound.”
DiMaio points out that there are some differences between full metal jacket 5.56/.223 rounds and 5.56/.223 rounds with soft tips or hollow points. He refers to these rounds as “hunting bullets” and reports that they begin to expand or mushroom shortly after entering the body.
He states, “a large temporary cavity is formed almost immediately on entering the body. This is augmented by shredding of the [bullet’s] lead core.”
The same type of injuries described above with full metal jacket rounds occur with the use of soft point or hollow point 5.56/.223 rounds, but they happen almost immediately upon impact with the body.
