I'd leave out the stain. Walnut usually looks good enough raw. As far as tung oil vs. poly, what are you looking for in the finish ? Tung oil will always require maintenance, as well as a long time to initially finish. I think it has a 'depth' to it that brings out subtleties in the wood. Poly is much more water-resistant, and will be done pretty quickly. It looks nice and shiny, but is different than tung. I also think its slick for a rifle that gets carried a lot.
My preference is a drying tung oil, but I don't mind its limitations and maintenance.