I want to shoot some steel at ducks
Well the steel wads are a bit different than the basic reloading wads. I have tried AA and Remington plain wads, and they do melt. FYI soap and water, NOT! That's a function of the smoothness of the barrels, so in the previous post, your ability to remove plastic wll be directly related to the smoothness of the interior of the ML barrel. If soap and water worked that person was indeed fortunate.
Second, there is a big dif between using a BP shell in a breech loader, and ramming a load past plastic fouling. Knock some of that fouling loose with the ramrod movement down and back, and you may have a barrel obstruction. (Use a felt wad to protect the shot cups is again, a very good idea.)
Solvents..., original Hoppe's #9 had Benzine, which would melt the plastic in one swipe, which is how it made its name...., but Benzine causes cancer, so OOOPS, but they refomulated it after the use of BP and Cordite had dropped off in sporting shells so the lack of the Benzine didn't hurt the product so much. I tried copper, I tried bronze, and I tried steel brushes on my shotgun, the plastic was still there. Finally I had a chemist friend who had given me an vintage bottle of Hoppe's, and (when I found where I had put it in my basement) I got the plastic out. Best is to avoid the plastic fouling all together.
LD