Maybe the reason the Colt ammo shot better is the fact that most 45 ACP ammo has bullets that are .451 dia and the colt is .452dia.
I think it's more to do with the significant amount of freebore in the 45 Colt chamber and that unsupported bullet slamming into the edge of the throat on it's way to the forcing cone. Convertible 45 single actions tend to have better accuracy as they have cylinders specifically made for the acp cartridge.
