As a footnote to my post of yesterday, I have found that by going to a harder bullet helps with the feeding. While 20:1 or 30:1 are standard black powder bullet alloys, straight wheel weights or Lyman No. 2 alloy will give harder bullets that feed smoother. Granted, if you are using this as a hunting round the expansion will not as much as with a softer alloy, but with a bullet this big in diameter how much expansion do you really need?
Try a harder alloy and play with the overall cartridge length, as Tuolumne Lawman mentioned, till you get a round that feeds smoothly. Once you decide on an overall cartridge length, you can determine what powder charge of black powder will give you the powder compression you want. I don't think that the idea here is to cram as much powder into the case as will fit but rather to get a reliable feeding round with an appropriate compressed powder charge. After all, we are shooting these guns for pleasure, mostly at targets, and not in a combat situation.
HHW