Beats me.
A 20" barrel on an AR only gets you another 50yds, or so, of frag range in comparison to the 16".
There was an AAR posted on here a couple months back saying that the typical rifleman kill was, on average, about 37yds in Iraq, and the average Sniper kill was about 75, or so.
Well within deadly for your average .223, IMO.
WIZZO
BTW, I love .308, though.