You could shoot a naked ball out of the rifle, but what would be the point? Accuracy would be non-existent and the barrel would foul badly and fast. Jeff Tanner in England will make you a mold well, and cheaply. If the bore is .270", have a mold cut to cast a ball .260" - .265".
Your barrel is likely made of iron rather than steel, though I may be all wet. If it is iron, your substitute powder's fouling will destroy the barrel in quick order. Non-corrosive...BULLFEATHERS!!
You asked for advice. Don't discard it because of expense, or because of advise from someone trying to sell you something. For the sake of a beautiful, intact, well preserved by your two preceding generations, and valuable rifle, use black powder - not a substitute.