The gas-system is the kicker.

My AR-10 is in 260 Remington, with a 22" Lilja barrel and Bennie Cooley brake.

JP extra-long AR-10 forend tube.

I've got a Leupold 3-9x40mm PR scope on it now. I think I'd like to replace it with a Nightforce 3.5-15x though.

I had a 6.5-20x40mm on it, but it seemed dim at 20x, and the mirage was horrible on prairie-dogs @ 500yds (@20x). It was also very sensitive to head-position / eye-relief.

I read a magazine-blurb by David Tubb once, and he said something similar that 15x-16x is about as high a magnification as you can go (for the particular type of shooting he was talking about).

The Nightforce 2.5-10x24 NXS looks like a kick-butt scope, but I haven't played with one.