Here's some info I found researching this same issue for a buddy of mine. He went with the 6.8 SPC. Please look at the attachment IOT compare ballistics.
6.8 SPC-- The .30 Remington is the parent case. The case was shortened, given a new shoulder and blown out to maximum capacity for a new high-performance propellant.It's basically a .270.
6.8MMx47 REMINGTON SPC BALLISTICS
BULLET VELOCITY (fps) ENERGY (ft-lbs.)
Muzzle 100 yd. 200 yd. 300 yd. Muzzle 100 yd. 200 yd. 300 yd.
115-gr. FMJ 2,800 2,523 2,202 2,017 2,002 1,622 1,250 1,039
115-gr. BTHP 2,800 2,535 2,285 2,049 2,002 1,644 1,345 1,075
115-gr. MK 2,800 2,535 2,285 2,049 2,002 1,644 1,345 1,075
*24-inch Barrel