What caliber to choose?

I have a .50 caliber Dragon Slayer that I had converted to .357 with a barrel insert that I have not shot much. I converted it because I was unhappy with the .50 accuracy and I have really not shot it enough to know if it is an improvement. I purchased a Talon with a custom shroud and 24 inch .20 caliber barrel at the Baldwinsville show this year that is a tack driver. I talked to Eldy about a 30 inch .357 barrel but am considering the .457 also. I have .357 ammo so that is an advantage in staying with that caliber. It would be just for fun and target shooting as here in PA. hunting is not permitted for anything with an airgun. I would like to see how far I can accurately reach with a big bore.