A stated already. AR500.
3/8" or 1/2" preferably for a 338. Distance is your friend, I ask guys shooting something that big to stay away from anything under 500 yards. Depending on your mounting system, 300 yards is doable, but it's hell on the hardware and shortens the life of your steel.
I use 1/2" grade 8 bolts and blue 3/8" rigging chain hung from drill stem pipe.