Can't help you on the barrels.
Best bolt action .308 powder is H-Varget
Gas gun powder, Chad turned me on to IMR 8208 and it creates some nice velocity in the AR platform. Varget's burn rate means it would produce a slower MV in the AR. I loaded the 155 gr. Sierra Tipped Match King to gain velocity while using a bullet that has a higher BC than the 175 SMK or 178 A-Max. I would walk the same path again, I actually got the combination correct.