What is the case capacity of the 260 and the 6.5x284 in a short action? What is the velocity difference? I am not going to use the 140 grain bullets.

I have a custom action coming and want a rifle capable of shooting 600yards, which is the longest my range is. I will also be using it for hunting. (I will not be shooting at animals that far out.) I was thinking a fluted #4 barrel at 26 inches should give me a good split between being heavy enough to shoot easily but still able to carry. Dropped into a McMillan M40A1 stock with standard fill should round out the build. Would anyone suggest anything else?