I have 2 Opentops, one is 44 Special and the other 44 Colt, in both I shoot 44 Russian. I also have barrels and cylinders to change them to 38 Special and 45 Schofield.
The 45 Schofield rim barely clears the ejection port, and the cylinder is thin, but it shoots fine. The 38 Special is a good and accurate caliber in the gun, but imakes the gun heavier and doesn't balance as well.
My favorite is 44 Russian, and a load of 18 gr BP with a Black Dawge bullet is smooth, smokey and light of recoil. The 45 Schofield kicks less than a 45 Colt, but not by a lot.
For the price of a set of dies, the 44 Russian, which will chamber in both the 44 Special and the 44 Colt cylinder is a very good cartridge for CAS.