Dixie Gun Works sells cast replicas of proper Enfield type minies, they look like artillery shells without any grease grooves in them. Just a hollow base. The bad part is that they are not cheap. You can find sort of generic musket minies at gun shows from bullet casters etc. that are sort of the typical Springfield type 3 groove minies for much less. How well they perform in your rifle depends on alot of things, like the lube the powder load etc. I have a bag of the generics for a musketoon but have not gotten around to trying them so I cannot really help you on specifics. I believe the authentic Enfield bullets also had wood plugs in the hollow bases but I doubt that the new ones have that. Otherwise I suppose round ball would work if you select the right ball size and patch thickness. Well presumably the sights are regulated for the minie, not the lighter balls? Depending on whether or not they take that into consideration making the replicas. Dixie may have loading data for the rifle too. Their catalogs have this info and their website might also. 70 sounds like a full charge, you might experiment starting 10 grains lower than that.