What do you intend to hunt with the rifle? A short barrel 338 is more than enough gun for anything in North America, seriously. I'm putting together a FCP-SR in 450 Marlin right now just because I wanted a nasty short action with detachable magazine. You could put the 450 or 458 in your long action if you think you need it. For the cost, I would just get another barrel if you want another caliber, that way you can switch them if you want.