I assume that you are building it off of an AR10 platform. In that case I would go with the 1:8 twist barrel, which will stabilize the 140 grain bullets a little better. I would also go with at least a 20" barrel. This will keep the velocity up. But this all depends on what your plan for this rifle is. Good luck.
Yes it is an AR-10. All I have yet is the lower receiver. i dont plan on hunting with it. I just want to build an AR-10 and want to do something other then a 308. Original plan was a 260 Rem but Im leaning toward the Creedmoor now. Not alot of barrels avaliable and xcaliber is the best deal I have seen but alot of options to choose from. Thanks for your input!