Bear, I have two 16" barreled ARs with 1/9 twist rates and 69gr is the heaviest (actually 68gr) round that I've been able to get to shoot accurately...I usually list 69gr since 70gr are the next step that I tried and they were miserable at anything other than close range (50 yards)..
Like others have suggested, if you want to shoot the heavier 65gr and above on a regular basis, I would opt for a 1/8 twist barrel...If any of your shots will be at the longer ranges, I think you will ultimately be more satisfied with the performance...