If what I suspect is true, lapping the bore could never get the job done. I used emery cloth on one and even though it smoothed out, it was still pretty rough. I still retained some rifling and it did shoot and cleaned better. You go through a bunch of patches to get it half-a$$ cleaned. I'd say do your best and if you are serious about shooting it, get a new barrel or spend a bit more and get it sleeved. Grandpas are very special and understanding !! ...
Be Safe !!!