Like quick said Birchwood casey tru oil works just fine. Straight out of the bottle, I rub it on with a finger tip and again, apply several coats.
Defenatly stay away from PolyUrathane!! Yes it's qiuck and easy but IMO absolutly ruins a fine wood!
I like the end result with linseed oil better than tru oil, but a proper application of linseed oil can take months with further applications every year so to maintain the desired finish. It takes a looong time to dry between coats. Tru oil can be applied in light layers twice a day and a nice finished product can be had in a week.
Just to throw a wrench in the mix, I'm building a ML in curly maple and will use Permalyn sealer applied as a saturated primary coat, then several coats of Tung Oil, but this gun will see little feild service and be a designated "Range Rooster".
All my field guns or those that are referb'd for freinds that will see field use have been done with BC Tru-Oil, it's easy to use, tough, and can be easily "touched up" in the future if needed. A few sandings when fully dry between coats fills the grain nicley,,Walnut is going to need some extra rubbing to fill the grain, put it on kinda heavy, then rub across the grain as it gets tacky,,another tip, When ya get the bottle just poke a small hole in the foil seal and pour a little out into a dixiecup or something,,If ya open the whole thing it'll start to dry in the bottle and make you sad.