A good 4.5 X 14 X 40 is my favorite scope for an AR. I prefer the Leupold Tactical. Another decent scope for the money, is Sightron 4.5 X 14 with side focus. A good 3.5 X 10 X 40 is workable. I just prefer the additional magnification that the 4.5 X 14 offers. How far are you wanting to shoot with your AR?