Nice 3rd Model. I have a Blue one in similar Condition. Definitely .38 S&W Ammo only. Personally; I would only use Handloaded ammo with about 10gr of FFFG Black Powder and clean it thoroughly after use. Starline make the Brass and Speer and Hornady both make HBWC lead in .358 which will work fine and save you a packet. It's a 130+ year old Gun that wasnt made for Nitro Loads; BP will be much gentler of your ole girl.

Just my 10 cents worth.