There is more going on with paradox barrels than just a few inches of rifling. There is a short choked section just in front of the rifling and built in convergence in the assemblage and construction of the barrels. Paradoxes did shoot well with both shot and conical ball. During the time Holland and Holland's patent was in effect many other makers introduced different methods of rifling or barrel construction to produce the same results as the "Paradox". None were as effective as the original paradox guns.
If you don't already have it you need to get Roger Lake's book "Paradox".