I found this editorial by Frank Plumb to be thought-provoking and information. He should have disclosed he is the owner of Handl Defense which has, obviously, a vested interest in seeing the use of the SCAR-H and 7.62x51 cartridge used more widely in the US Military, but I have heard from numberous combat vets who have experienced similar results with the 5.56, stitching a bad guy full of them and not stopping him, etc. I know it is an ongoing debate, but this does offer at least a well thought through set of arguments for a heavier bullet. Full op ed here.

Excerpt:

The way a bullet destroys muscle, solid organ and soft organs is different. This is based off the cavitation effects of the projectile once it enters. Imagine looking down on a boat moving through water. It leaves a wake behind it. The size, shape, and speed of the boat determine the size of the wake. Think of the boat wake as projectile cavitation.

Since Humans are about 70% water I feel this is a very accurate metaphor. Look at the wake of a speed boat, like the type that are raced in the open ocean. They leave clean small wakes by comparison to their size. It the pursuit of speed, they reduce drag, they transfer as little energy into the surface of the water as possible. This is akin to how a 5.56 projectile works. Clean and fast through the air often means clean and fast in the target. According to Army studies a 5.56 bullet needs 4.75 inches of body penetration to Yaw. This Yawing is critical to the 5.56 round being lethal. It is how it transfers its energy into the target. If the 5.56 round fails to yaw, it often fails to kill. That August night in 2003 makes a lot more sense now.