One could use satin or semi-gloss polyurethane, but IMO poly is a less than desirable finish for gun stocks. It is harder than spar varnish and will develop 'crazing' if the stock is subjected to extremes of climate change or dampness. Wood will move, no matter how well seasoned it is, and I prefer a finish that will move with it (like spar varnish which is more elastic than polyurethane).
Realizing that the following statement doesn't address the OP's question, I fall into the camp that prefers a glossy spar varnish finish, sanded between coats to achieve a glass-like surface (usually 8-10 coats on black walnut)- then rubbed out to dull it, and waxed to restore a 'sheen' rather than a gloss. I've had this method claimed to be an oil finish by unsuspecting souls.
