I was wondering about that too...found this...
5.56 NATO (.223)
The 5.56 NATO (.233) is the work horse of the American Military, as well as the armies of many other countries, being the cartridge used in the M-16 Rifle, first adopted during the VietnamWar and now used in the weapons of many other countries
For a little bullet, the 5.56 bullet produces quite dramatic wounds. While the traditional 30-06 caliber bullet of the M1 Garand and 7.62 bullet of the M14 rifle would immediately knock a man down, the 5.56 bullet instead enters the body, quickly turns sideways after passing through only 4" of flesh, then breaks in two major pieces, as well as many smaller fragments. During the Vietnam War, soldiers reported that shooting an enemy soldier with the M16 did not kill as quickly as the old 30 caliber weapons. Instead soldiers would follow a massive trail a blood a few feet away from where the enemy soldier had been hit to find him dead from massive blood loss. This light-weight cartridge permits soldiers to carry more ammo, but is not as effective at long distances as heavier cartridges and does not penetrate steel as well. The low recoil permits quick follow-up shots and minimal muzzle climb during automatic fire