Actually, percussion caps used to come in a container marked "centerfire". Cap and ball revolvers are centerfire guns, not muzzleloaders. They are discussed on the Forum because they existed at the same time as muzzleloading caplock rifles, and were in general use.( Or so memory serves. I don't have time to look up the Rule and lose what I've written.) The conversion cylinders fire modern, brass cased cartridges with self contained centerfire primers, which is why they are not discussed here.
JamesA, the Cimmaron convertible sounds like a fun, useful revolver. If you want it, buy it.
here is an example. Look at this cap tin in Griswold and Gunnison cased set.