The Cap and ball revolvers can be converted to shoot cartridge rounds. However, something to make sure of with them. Make sure it is a steel frame. They have alot with a brass frame that are labeled as confederate models. These are NOT strong enough to shoot 45LC rounds. Blanks maybe but for the cost not worth it in my opinion.
The R&D conversion has 6 chambers for 45LC on most of their models. Depending on the gun you get. Ie. Remington clone, colt clone. This would allow you to have your empty chamber.
Kirst Konvertors have 5 chambers for the 45LC however they have a dead spot or dummy chamber for the hammer to sit. This is legal for Cowboy action shooting, I would assume for mounted as well. The cylinder has to have this empty spot to ensure the timing of the cylinder is right.
I have several of the Cap and ball revolvers and they are a blast. However I'm not sure they would be your best choices for mounted shooting.
The colt clone 1851 I have has a 7.5 inch barrel. $189.00
The 1858 Remington has an 8 inch barrel. $209.00
Then you add the conversion cylinder. R&D $240 Kirst $290
Then the smithing to convert over to a Kirst gated cylinder. Around &200 bucks.
After everything is said and done you've dropped a pretty good chunk of change when you could have just bought a nice Ruger.
The Uberti's are nice guns. Cimirron and Taylors are both reworked ubertis. Just keep in mind that the Ubertis are built off of the original 1873 SAA specs. Which means they have the same draw backs. Which I'm fine with because I like the historical side of these firearms.