The two most well-known cartridge conversions of the Colt were the Thuer (not very many made) and the Mason-Richards (much more common). While dual system guns did exist they are not all that common now. Getting the hammer to strike (a) caps and (b) primers interchangeably must have posed some problems.
Various lookalike cartridge revolvers were made by the Italians in the '50s/60s.
Today here some people are converting Ruger Old Armies to smokeless muzzle loaders, using shotshell primers.