Re: **338 Caliber Choices**
If the action you have won't handle a RUM, the 338-375 Ruger would seem to be a good alternative. My understanding is that it will get you .340 WBY-ish performance.
I know elkaholic did some kind of wildcat in a .30 caliber on the .375 Ruger case. He might be a good guy to consult regarding the ins and outs of a .338 version. Might have some helpful hints on a reamer and dies, too.