I've always been a proponent of spar varnish on gun stocks. I spent half a lifetime in the yacht carpentry profession and came away with a lot respect for the stuff. I typically apply about 9 coats on a stock, sanding with 320-grit between coats, ending up with a glass smooth glossy finish. If a lower luster is desired, rub it out, then wax it. More than once I've had a rubbed out spar varnish finish mistaken for an oil finish. If anyone uses spar varnish, get the best you can find. I like Epifanes but it is admittedly hard to find and expensive, but OMG does it go on nicely. Gloss has a higher UV rating than semi-gloss or satin, that's why I use it and then rub it out.
Another advantage good spar varnish has over polyurethane, aside from the generally better UV protection found in the hardware store brands of poly, is it's elasticy. It doesn't get as rock hard as poly and hence resists cracking/checking better IMO. Remember, it was originally formulated for use on wooden spars that bend a lot in use, in a salt environment.
The thing with sprayed on hard clear automotive finishes is that they are formulated for use on steel which isn't as dynamic as wood, and may crack over time as the wood expands and contracts with changes in humidity (and it will, no matter what you use for a finish, to varying degrees). I may be all wet with that, but it's what I've been told by several professional finishers.