Between 338 RUM, 33 Nosler, 300 PRC and 30 Nosler I would see which one I could get really good brass for and go with that, and I would do the 338 over the 30's I think. My guess is that would land me on 338 RUM. If I were doing a 30 cal I think I would be hard pressed to not go with a 300WM and more so if the shots are 500 and in. Good brass is relatively easy to get, plenty of horsepower too and in my experience a 300WM is pretty easy to get shooting well. Another plus would be factory ammo is relatively easy to get if for some reason you needed it.