Ugh. Guns stocks and stains. So much harm done.
The wood's got great color already due to being a) walnut and b) old. Stains are a lot trickier to get RIGHT than almost anyone (especially on gun forums) seems to believe, and an average stain job looks like (I am sorry, but) crap to anyone used to high quality woodworking.
(Stains tend to concentrate in the more open grain areas and not absorb in the denser areas, making for a zebra effect which is unnatural and UGLY. Further, the pigments overload the grain making it look muddy and blotchy, taking all the fire and shimmer out of the natural figure of the wood. Much better to go with a completely natural/neutral finish and let the wood darken on its own over the next decade than to muck it up with an amateur stain job.)
The top coat looks rough. As others said, there are solvents that might remove it pretty easily without doing anything nasty to the wood.
I'd second keeping the sandpaper off the gun. It does have a place, especially if you keep to the grits above 220, but it can also do harm very easily. Of course, sanding off edges is bad, but almost worse is cutting through the natural patina (that beautiful rich color) that the gun has spent eighty years developing).
I'd clean off that top coat and then do a few applications of TruOil, Tung Oil, Watco (my favorite), or linseed oil. No stain. Maybe a very light pass with fine abrasives in between coats. The gun will look incredible.