Oil finishes like tung oil, boiled-linseed, Watco Danish oil, and the like are great for adding color, and protecting the wood from water but they don't give you a hard shell finish, so the won't protect the wood from abrasion and they won't really fill the pores of the wood. Also, unless the oil says it has color, don't expect it to appreciably darken the wood.
Polyurethane isn't bad, but it it's a little soft. It does however give you a gloss (if desired) and it gives a thicker shell finish. Each layer of poly is distinct though, they don't bond together.
Shellac and such is what high-end furniture makers and woodworkers consider to be "best". It gives a hard shell, it can be tinted, you can tweak the gloss, and each layer bonds to the lower which gives a deep buffable finish.
For a knock-about rifle I'd give it 2-4 coats of oil, wait about 3-4 days, then hit it with a couple coats of spray poly with gloss level of your choice.
A show piece, well that would take some time to write up and I don't think you're planning for that.