Most furniture is finished only on the outside exposed surfaces. Open up a chest of drawers made 100 years ago or 200 years ago and clearly there was no finished used on the inside surfaces. It seems finish is only needed on surfaces exposed to the elements.
Pause and consider the world of about 1900, those guns and furniture and other finished wood articles were born to.
The Morgans and the Vanderbilts, didn't have air conditioning, and had no way to regulate the humidity in their homes in the summer. The wealthiest people with electricity might have had a small electric fan. In the winter, wood and coal heat sapped the humidity out of the indoor air. Gun safes, were closets. Closets, often were pieces of furniture called wardrobes.
Guns were transported to the fields and trap arenas in carriages, buggies, on horseback, and on railroad cars. The best express cars were heated, but none were cooled or humidity regulated.
If there ever was a need to seal wood on the inside of furniture and gun stocks to the elements, our great great grandfathers had a reason to do it. The biggest difference between where the furniture and the guns slept and the outside was there was no rain, snow, or bitter cold inside homes.
Maybe, it was such a losing battle they didn't make the attempt. Super glue and polyurethanes were way in the future, and mostly they had boiled linseed oil, shellac, lacquer, and varnish, to finish the outsides of the wood. Those weren't really true sealers against the elements, even had they finished the inside of the wood.
And also consider the care that had to be given to any firearm that survived in decent shape for us to own. All ammo was corrosive. Synthetic gun oils didn't exist. When a gun got wet, it had to be taken apart and inside from the wet, and dried out and oiled, or it became a rusted mess.
Even forty years ago, well into the WD 40 era, when I was away at college for even a month or so, rust might start to bloom on the metal parts of my guns hanging in a farm house with no air conditioning, and wood heat in the winter.
So not only did it save labor and money to leave the insides of the wood products unfinished, and it was the traditional way to do it, even if our ancestors had tried to seal the insides, they didn't have the ability then, to even seal the outsides of the wood. The finish, was just that, a finish given to the walnut, pretty to look at, and not anymore proof to the elements than the owners.