Howdy
1. Do not even think about putting any type of Smokeless powder into a Cap & Ball revolver. Never! You may not like the thought of cleaning Black Powder fouling from a revolver, but I am here to tell you it is a lot easier than most realize.
2. Colt never put an octagonal barrel on the Single Acton Army. While there are modern 'replicas' that have octagonal barrels, Colt never put one on the Single Action Army. They were always round barrels.
Like this:
3. There are conversion cylinders available for most Cap & Ball revolvers. These cylinders are proofed for Smokeless ammunition, so even though the gun may say 'Black Powder Only' on the barrel, it is safe to shoot mild Smokeless loads in these.
Here are the cartridge conversion cylinders offered by Taylors.
https://www.taylorsfirearms.com/hand-guns/conversion-cylinders.html
This is my EuroArms 1858 Remington that I bought many years ago. A few years ago I added a cartridge conversion cylinder to it.
This particular cylinder is only marketed by Taylors Firearms. It is the only cartridge conversion cylinder for the 1858 Remington that has six full sized 45 Colt chambers. Although I usually shoot it with Black Powder ammunition, the cylinder is proofed for mild Smokeless ammo. Yes, it is more expensive to buy a Cap & Ball revolver and then buy a cartridge conversion cylinder for it than it is to buy a cartridge revolver in the first place. In this case I had bought the revolver so many years ago that adding the cartridge cylinder was like getting a new cartridge gun for a little bit over $200. These cylinders cost a little bit more than that now.
This 'cartridge conversion' revolver leaves the factory as a cartridge revolver. It it based on the historical conversions that were done to the 1858 Remington during the Civil War. However with this model you do not have to buy a Cap & Ball revolver and then buy a conversion cylinder for it. It is a cartridge revolver from the start. The frame is slightly larger than the Cap & Ball replicas, so the cylinder is not interchangeable with the C&B models. This revolver is available in several calibers, including 45 Colt, two different barrel lengths, is proofed for Smokeless ammunition, and they all have octagonal barrels.
https://www.taylorsfirearms.com/hand-guns/cartridge-revolvers/1858-remington-conversion.html