The Pietta 36 is a 6-1/2 inch barrel. They probably started making a 5-1/2 inch 44 to counter the one made by Uberti. I have the 5-1/2 inch 44 Uberti and like it very much. It is a pretty light weight revolver, and a 45Colt in the R&D cyinder kicks pretty good if it has a 250 grain bullet. I load lighter with a 200 grain bullet, figuring the rifling twist is slow for a roundball, and the shorter 200 grain bullet should shoot better.
I also love the 36 Navy Remington in 38 Special, and I load 38 cal 148gr Hollowbase wadcutters. Remington and Speer don't lead as bad as the Hornady bullet. It's hard to get enough lube in any of those HBWC bullets, though, for black powder use. The Rapine is probably better, but in a 51/61 Colt would have to be loaded in a 38LC case.