I haven't sanded any gunstocks I've refinished in the last ten years, except for repaired areas.
That's when I discovered Formby's Furniture Refinisher (liquid) at WallyWorld - where you can also pick up a small can of Min-Wax stain/sealer in the darker color of your choice, along with a package of fine OOOO steel wool pads.
The same products are also carried at many good hardware stores, Home Cheapo, and Lowe's Home Improvement Centers.
Follow the Formby's can directions, to wipe off the old finish with a steel wool pad, let it dry overnite, stain/seal the wood, re-stain if it's not dark enough after the first session, and apply a final finish, like Tru-Oil.
If there's checkered areas, use two different old toothbrushes on those areas - one to strip the old finish, and the other for the Tru-Oil.
.