Before Elmer Keith for into hot roding the .44 special he first tried to soup up the .45 Colt. After destroying a couple of single action Colt revolvers he finally concluded "no smokeless powder load safe to fire in Colt revolvers will equal the power of the Remington blackpowder factory load". Today we have many powders not available to Elmer back then but it is still good advice to keep smokeless loads short of blackpowder velocities. I have a Kirst Konverter, the five shot gated conversion, on an 1860 replica and the outside chamber walls measure only .033" thick. That's pretty scary business and you bet I keep loads light. That is why original Colt conversions were in .44 Colt caliber and why the modern conversions are five shot, .45 Colt just won't fit in a six shot cylinder in an 1860. I'm very impressed that Colt was able to shoehorn six .44's into that rebated cylinder. The Remington is a larger cylinder and a stronger frame, but still not a Ruger Blackhawk or even a new vaquero. In working up loads for my Kirst converted 1860 I started with loads under 600 fps and worked up very slowly. The hottest load I've tried so far is a 200 grain bullet just shy of 900 fps.
Another consideration is rifling twist rate. The cap&ball revolver was designed to fire round balls or short conicals. My Pietta 1860 has a 32" twist, the standard for .45 Colt is 16", or twice the twist rate of the C&B revolver. I tried a few 250 grain SWCs and RNFPs from my Pietta and they were all over the paper. My best groups have come with .457" round balls but 200 grain RNFP are not too bad at about 2 1/2" groups at 25 yards.
