Something to remember is SMR’s were called black rifles long before modern man started making gunstocks out of milk jugs,and old tires, and calling them “Black Rifles” . A likely grain filler would be asphalt, or tar, devolved in turpentine, to a consistency of grandma’s gravy. I’d wipe it on a final sanded stock, and then wipe it off. It will likely take a couple of applications to fill the pores of the wood. The process will no double color the stock to some degree. So, you will have to decide to either stop there, and apply a finish. Or, go over it with a stain to even it out, and give it a bit of a tint, and then apply a finish. A tinted varnish could do the evening out, and the finish in one step. I would rub any varnish back a little to cut the shine, and then wax it good with paste floor wax.
Hungry Horse