If I were just getting into the BP idiocy... (my kids used to call my work "Dad's idiot projects"):
I'd buy or check out at the library a copy of the book by Warren Ripley, "Artillery and Ammunition of the Civil War" and from there I would send $ to AOP (Antique Ordnance Publishers) and purchase the drawings for the piece(s) I think I want to buy. That way I will see exactly what the originals were, I will be armed with more of the right questions for any potential vendor, and I will substantially reduce the risk of being suprised by what gets delivered in relationship to what I think I had ordered.
A few more (but lesser) $ on the front end... but much greater assurance of being fully satisfied and NOT suprised with the delivered and subsequently fired product.
Also:
What the original was in terms of looks and bore, you will need to translate for yourself what that means after a builder has scaled everything down to shoot soda cans or golfballs. Scaling a Seacoast vs scaling a Coehorn... to assume one will be bigger and better because the original was a huge seacoast gun and the other a two-man crewed field gun may very well turn out to be a false assumption. As a builder myself (for myself) I will scale the bore to one scale and the exterior to another... For example, for a pool ball mortar copy of a Coehorn, I scaled to .4 for the bore and powder chamber but raised the exterior scaled dimensions to .45 to both fit some 4" stock and to give an added margin of metal/safety. My walls are just as thick it not thicker than a scaled 13" Seacoast when going down to a golfball sized mortar.