I used to be up on this stuff, but that was 25 years ago. All I know for sure is the 5.56 mm is probably not the best long range military rifle. But around here, 100 yds is a long shot because of the heavy cover. The 5.56 mm seems to work well for police in urban situations as well. If you find an expanding round that is reasonaby accurate in your weapon at 200 meters, that should be good enough for any civilian defense situation I can imagine. If you get a head shot, it really won't matter what kind of bullet you use, your attacker/enemy will be shut off and DRT. I am loaded with 64 gr Win Ranger Power-Point Law Enforcement ammo. That should do the job in any of my .223s. For home defense, I use a 20 ga Mossberg loaded with #1 Buck shot.