From what I understand, a Pietta needs at least a .451 ball, I use .454 diameter in both Piettas and Ubertis, the Uberti chambers are a little bit larger. I have a few Uberti cylinders that require me to use the Ruger .457 size, but in a Pietta or tight Uberti the .457 could bend your loading lever.
Conical bullets are made by Buffalo Bullets, but might not fit under the clearance cut on all Colt types. I have used Speer 200gr SWC bullets made for the 45ACP, but they are a tight fit on the Colt, fit more easily in the Remington frame.