You need to know what model it is, 1st and 2nd models are black powder ONLY. 3rd models can take smokeless. Can you post some pics?
I love the old Breaktops. When I'd work cows I'd have a .38S&W in my pocket just to have some kinda gun. They are all old. They were made with metal that just isn't as tough or strong as that we have now. To be on the safe side, IMO, use black powder in all of'm. It's easy to load, really easy to clean up and about as safe as you can get.