$599 is almost half off rack rate. Well done!
I don't recommend shooting ball out of a 20 ga barrel in a pistol. You can shoot buck - bird from it though. For S&G, I would use #7.5 shot following the 50/50 rule, powder to shot by volume. If you are getting poor combustion, I would work first on gas seal, then possibly drop the powder charge. I would start at 11/16ths oz and work up to 7/8ths, which may be a bit much. Were you to go afield against other armed horsemen, I would load with #4 buck. The key to success with shot from such a short barrel will be gas seal and over powder pressure. You need a heavy fibre over powder wad a good .005 - .010 overbore seated with considerable pressure to get anything like full combustion and any velocity. If you want any kind of pattern past 10 ft, I would also use a stemless plastic shot cup. A good strong cardboard overshot card, well overbore, should hold shot in with no need for sealant, but that will take some experimenting.
I can't give you any advice on cylinder loads as I am unfamiliar with their volume.