Kabar2, Allen, is certainly correct here, Owen, in that there are quite a few different rifling types currently in use. What is really fascinating about your photo of the dueling pistols, is that neither Mike nor I have ever come across anything like that poly-groove, pointed-land, type in any book on artillery that we have. I may have seen an image of this rifling type in a book on antiquities that I once studied, but that volume only delt with small arms, not cannon.
The closest form to that used in your pistols is the 1860s rifling form used by Armstrong in the RBL-6 pdr. If I remember correctly those tubes were rifled with a poly-groove rifling which had lands which were narrower than the grooves, about .100" VS .150" and about .40" deep, which made this form very noticeable back in the days when 3 or 4 grooves and lands of equal width was the norm. Some of these artillery pieces ended up in Australia and New Zealand during the 1860s and 70s, I believe.
To take a closer look at the dueling pistol muzzle, we highlighted the photo, right clicked and "Saved-As" to a file we call "Photos ready for Fototime". Once opened, the photo automatically was bumped up two standard sizes which revealed the pointed-land, polly-groove rifling form. Further enlargement blurred the details noticeably.
Thank you both.
Tracy and Mike