I put a hand rubbed( read that as " stretched) stock finish in the lock mortise, barrel channel, and ramrod channel. I also do the end grain under the buttplate. and any mortise for the trigger guard, escutcheons, end cap, etc. That seals the wood well from anything that might spill on it, including water, and oil.
You can also use water pump grease in the barrel channel to protect the barrel and wood from taking on water. But, I would coat the wood FIRST with a good stock finish, like Birchwood/Casey True Oil.
I am not a big fan of shellacs, or mineral spirits. Shellacs do seal the grain completely, and don't allow moisture out either. They don't take sunlight well at all, as the UV light will cloud them in a New York Minute! Mineral spirits tend to keep raising grain in the wood, so that you are either forever going over the stock with steel wool, or sandpaper. I finally gave up using mineral spirits on gunstocks, and use scrapers now to cut off the whiskers, rather than steel wool or sandpaper. The scrapers do a better job, and it only takes practice to learn how to leave a fine, EVEN finish to the wood for staining, or coating with stock finish.