Generally speaking, you can apply an oil type finish (polyurethane) on to another oil type finish, so Poly over Tung Oil should work ok. But...I've never tried it. A lot of Tung Oils sold aren't really all Tung Oil, but are blends of Tung oil and Mineral Spirits. I do know that you could put a coat or two of shellac in the barrel channel and then a coat of Poly over that and you should be pretty darn water resistant. The reason I suggest Shellac is that it will stick to most anything (within reason), though it isn't truly water resistant.
But....to go the easy though pretty effective route, just use the Tung oil or Boiled Linseed oil or TruOil in the barrel channel. That'll do all you need done unless you are going swimming with the rifle. It'll be water resistant. I'd just soak the bare wood with Linseed Oil until it won't absorb any more oil and let it dry and you're done. You can add a small amount of Japan Dryer to speed up the slow drying process of the Linseed Oil. Soak any oil soak rags in water and keep them wet for a day or two to avoid spontaneous combustion. I know that from personal experience.