The barrel being less than pristine doesn't mean it wont shoot. My friend got himself a 22 (mod 60) that had been in a house fire and got wet and rusty from sitting after it had been drenched with fireman hose water. We "knew" it was to be a lost cause but cleaned it up anyway and I'll be darned if it didn't shoot better than my 10/22.
So the only thing you can do is to try and hope for the best. I have no knowledge of BP loads but I know that RX7 works very very well in many 45/70 loads.
"Always place your clothes and weapons where you can find them in the dark"
"Understand that the enemy is not the enemy in his own eyes ;this may offer you an opportunity to make him your friend. If not, you can kill him without hate, and quickly."