I have used Pro Custom Oil which is available through Brownells. It is labeled as a mix of tung oil and urethane.

http://www.brownells.com/gunsmith-t...custom-oil-gunstock-finish-prod5531.aspx

Not so much rain and moisture here in Arizona. Should one hunt in the PNW or AK, I suppose using the epoxy finish Art has suggested in the past, or switching to Alaskan walnut would be a better option