Keep in mind that with an AR-10 you an shoot accurately out to 800 or 1000 yards, but you’ll need the glass, hands and control to do so. That said, buy the very best scope you can afford.
Me, I’d be looking at Leupold. if you can afford it, otherwise, go with a Mueller or other well reviewed scope.
I have the Mueller AO 8-25x50MM on my long range 24” Bull Barrel AR-10 and I can hit stuff at 500 yards, 600 if I skip the coffee and really relax before I shoot.
I know. Unfortunately the gun club I belong to only has a 200 yard range.