I don't know a LOT about guns... but a 5.56mm Ar will drop a man out to about 500 yards, without much problem.
One thing I DO know about .226 AR's: They weren't designed (designed is the wrong word... they figured this out after there was a 5.56 bullet) to KILL an enemy combatant. The were designed to maim them. The .226 tumbles in the air, and rips great chunks out a person... but you want them to live.
If you drop a enemy, that's the end of that, one less enemy.
If you MAIM an enemy, it takes two guy to drag him out of the line of fire, then a highly trained medic gets involve, and maybe a highly trained nurse... to stabilize the wounded for transport... by helicopter, with two highly trained pilots, to a field hospital, to be treated by several highly trained doctors, to recuperate in a hospital bed that is in high demand....
One shot, one kill: you take out one comrade enlistovich
One shot, maiming the guy: You take 30-35 highly trained, VERY expensive personnel, and a bunch of very expensive equipment out of the fight.
THANK YOU Mr. Mcnamara ... ain't war grand?