DD:
I assume that is the Snider cartridge? You will get best results with a groove-diameter bullet between 480 and 560 grains. The original cartridge used the hollow-based bullet designed for the Enfield muzzle-loader, it was bore diameter (not groove diameter) and was a failure in a breech-loader because the skirt had stopped expanding before the projectile had fully exited the case (pressure was very low). The various improvements included clay plugs in the hollow-base to try to make them expand into the grooves, which sorta worked but not very well. By the time the Brits got around to finally admitting that a groove-diameter bullet was the only solution, they adopted the Martini Henry!

Slug the bore: you may have to get a mold made. My own Snider is a 3-band volunteer rifle by Barnett of London, and measures .598 across the grooves! There are plenty of recommended loads for the Snider, particularly HERE. Good luck!