Re: AR-10 w/ A2 upper
Brownells sells an extended A2 rail that mounts very low on the AR-10. I have an NXS 2.5-10 x 24mm mounted with low Warne rings and the cheekweld is very good. I drilled a second hole in the handle to allow mounting the rail almost flush with the rear sight at full elevation.
Might also try a standard leupold or other 1-4x variable, the smaller the ocular lens housing the better. You want your mount as low as possible. If this fits you, you will find the A2 fixed sights are better than the removable jobs and your scope will be very fast to use. Flat top is nice with a longrange scope, but really did you buy your rifle to be a dedicated longrange rifle or to be a multi-task weapon? If the latter, look into ways to hold onto all the benefits of your A2 system before ditching it...
An A2 AR-10 will easily handle 600 -750yd ranges with as much precision as a scoped A2 might try a NM rear apperture and tune your ammo to your particular rifle....