I-too would be trying real BP. Be aware that black powder will produce more recoil, but might be easier to find a series of loads that regulate than when using smokeless powder. This is what I found in my 12 bore. Use a good BP lube if you do. Alox (modern bullet) lubes do not work with black powder as they do not mix with and therefore soften the fouling. My BP ctgs. guns are good for 20 shots without having to wipe the bore, maintaining wiped-bore accuracy for the entire string.

SPG, Lyman's Black Gold and a 60/40% beeswax/vaseline(or neetsfoot or olive oil) mix works fine. Use enough oil to soften the beeswax mix for the ambient temps. It should be solid, but barely movable.

A lube disk will be eneeded for paper patched bullets to keep the fouling soft. the lube needs to be just above the powder, separated from it by a thin Beswax disk, or a simple wax paper disk works the best for me. The powder flame softens the lube and it mixes with the fouling allowing 10 to 20 accurate shots before wiping the bore might be necessary. Otherwise, the fouling gets hard and dry and cuts the patch off the bullets resulting in leaded and badly fouled bores.

BP should be compressed about 1/8" to 3/16" in the .577 case after being dropped into the ase through a long drop tube. Then the wads, then the bullet. Paper patched bullets need a BP lube to be applied to the outside - just rubbed on with the fingers in the same direction as the paper wraps on.