First off, what is the make and model of the rifle?
Don't do anything to the finish until you know what you have - you may reduce the value of a collectable to junk.
Take the action and barrel out of the stock, if there is a bolt remove that. The obstruction might be a dirt dauber nest if the gun left on the porch or barn, so to start, just pour some oil down the bore from both ends to soften the obstruction. After awhile, use a cleaning rod with a jag and patch to push the obstruction out. Clean the bore with solvent and patches, use a bronze brush if you have one.
After the bore is clean, wipe the entire gun down with light oil - 3 in 1, gun oil, aobut any petroleum product and let it sit for just a while. Then, take some 0000 steel wool wet with oil and gently rub the surface to remove the rust that stands proud of the surface. Try to leave as much of the original finish as possible. When you are done, you will have a gun with a nice aged patina. Clean the residue and crud off and oil the gun. That part is done.
Remove the trigger guard and give it the same gentle treatment.
Clean to stock with Murphy's oil soap or similar. Your goal is to remove the dirt and leave all the remaining finish, scratches and all. Give the stock a thin coat of Tru-Oil, let it dry, rub it out with a clean cotton rag until there is no haze visible in the finish a glancing light through a window, and stop.
Reassemble the gun and hang it on the wall; it will look great, and it will retain the scratches and dents from Grandpa's adventures with the gun.
Or, you can strip it of all finish, sand the hell out of the stock, cold blue the metal parts, and you will have a gun that looks Bubba'd up, but you can take comfort in knowing that you aren't alone. You also won't be doing your gal a favor.