Cosmoline? I wouldn’t use that unless I was going to store the gun for a few years.I just took a lot of scrubbing to get it clean, that being said there are probably better ways and means of getting these things clean, I am not familiar with the tricks of the trade so to speak. I just took some pre soaked "muzzle loader cleaning patches" and scrubbed until black stuff quit coming out. Then I removed and cleaned the nipples individually and oiled them and replaced them. Ran a patch with cosmoline down the barrels a few times and put an oily patch between the hammer and nipple.
I probably need some tools more specific to cleaning this particular gun, I'm sure that would help.
Patches wet with water will cut the fouling very nicely, then use a couple dry patches. Then a lightly oiled patch, followed by a dry patch to tidy up. Use a lightly oiled patch to rub the nipple threads before returning them to their home.
Prior to loading on your next go’round, wipe inside and out with dry patches to remove any residual oils.