You might get by using .45 bullets(.452) in a .44 C&B but they're not ideal. The lube grooves are too small, the lead is too hard, they're flat based so loading them straight will be difficult. Conicals made for C&B have tapered bases, wide lube grooves and are pure lead. Hollow based conicals are the most acurate.
I don't suppose it would hurt anything to try, given right size for your cal., but they should be cast from dead soft lead, not harder alloys like for smokeless guns. The hollow base on some helps due to the lower pressures of BP but solid bases are used in some BP rifle cartridges so I suppose it is a matter of experimentation. But the old designs work for sure.
