I answered you in a personal email last week, but there will probably be others interested in the answers for such a problem.
For revolvers with very long cylinder throats, always use gas checked bullets, and let me know about this issue when ordering a mold, as I'll design the bullet so it will grab the rifing harder after it's long travel.
As a rule, the slowest burning powder that will give consistent velocities, at low velocities, will give the lowest velocity gain in the cylinder, which means lowest impact speed with the rifling. With a short barrel though, slower powders which are barely burning clean, and perhaps not even burning clean due to the low pressures, will give a lot higher muzzle flash and bang than would a fast burning powder loaded to the same low velocity. Speaking here of speeds around 800 to 900 fps.