The cartridge was called 223 Remington cartridge later would be adopted by NATO with a heavier powder charge and grater muzzle velocity as 5.56X45 or 5.56 for short. Whatever the cartridges deficiency it has the distinct advantage of letting the guns user carry a lot of ammo compared to 30/06 or 308 or even 7.62X39. At fairly close range the bullet caries a vacuum behind it which on impact creates about 3000 psi in the torso and it would pay you to watch a ballistic gelatin video on the Internet in slow motion. The cavity is expanded and the damage is not just from the hole the bullet makes but by the inflation rips and tares creating massive internal damage. The bullet many times bounce around in the body surrounding tissue from bone to bone. So whether fired from close or longer ranges the bullet will ricochets around the body creating more damage along with the vacuum effect.
Richard W. Norman Author of American Handbook on Guns Ammo and Freedom