Polyurathane makes a very durable clearcoat. However, I would recommend Spar Urathane for your clearcoat, for a couple reasons. First, Spar is meant for boats, and it's probably one of the strongest and most waterproof clearcoats available. Second, it's more UV-resistant than most other clearcoats, meaning it'll keep your stain from fading in the sun. And it doesn't have a tendancy to yellow over time, like some other clearcoats can.
Having said all that, Spar tends to be a little more expensive than regular poly, but is worth it to me. If you don't plan on rough use of your rifle, regular poly should work just fine. Minwax makes a fine Spar (IIRC, Minwax Helmsman), and it's always advisable to stick with the same manufacturer of your finishing supplies if possible. This is a better guarantee that your stain and clearcoat will get along.
Other than that, and the above-mentioned "whiskering" removal, you should be good to go.
One more note though. Walnut is a tight-pored wood and as such, it is not recommended that you sand with anything finer than about 400gr max (actually, 220 is the best) because finer sandings will start to burnish the wood and seal the pores. If this happens, your clearcoat will not adhere very well at all and tend to chip off. But you can fine sand the clearcoat as fine as you want to go, depending on how smooth and glossy you want to get it.
One more thing (sorry, I'm almost done): Make sure you let your final coat of stain dry THOROUGHLY for at least 24 hours before clearcoating. Because most of Minwax's stains are oil-based, any wet oil will not get along with clearcoats so they need to dry completely. Don't rush it and your stock will come out beautifully.
BTW, once it's done, you should post a pic or two if you can, so we can all see it.