Hello Electric Miner,
Nice find, I'm sure you will have fun with that one.
I would imagine it's a .357 bore, so either 38 Special,
38 Long Colt, or 38 Short Colt with .357 dia. bullets.
Hello Richard,
This is not a Conversion, rather an 1871 Open Top Uberti.
I'm sure the chambers meets the SAAMI specs.
AntiqueSledMan.