Rich, depending on the range that you want to use your AR, I suggest that you might want to zero the scope tightly at 50 yards. By doing a 50-yard zero, all hits from zero to 250 yards or so will be within a few inches of the aiming point. The following list is the approximate errors for XM193 ammo in an M16:
5 yds -2.4"
10 yds -2.1"
25 yds -1.2"
50 yds 0.0"
100 yds +1.6"
150 yds +2.1"
200 yds +1.1"
225 yds 0.0"
250 yds -1.6"