An 18" AR10 style carbine is not much of a "Long Range" rifle, mainly because you are stuck with loading any round to the magazine OAL. I would send you the opposite direction and go 12 or 13 twist, that way you can shoot the 110 and 125 grain bullets. You will gain more FPS with less twist and not have extraction problems. You will still be able to enjoy the 150's and 165's with adequate 500 yard accuracy and thumping power also.