Hello the Camp
I am puzzled by the problem of melted plastic in the bore when, I suppose, using Gun Powder as opposed to using nitro Powder.
I have shot away literally tens of thousands of rounds of 12 and 20 gauge shells both factory and reloaded at skeet, trap and hunting in the past 50 years. Usually a couple of passes with a tornado brush and a patch of solvent and the bore is shiny bright.
My SASS Gun Powder load is FFg powder, card and fiber wad, shot and over shot wad closed with a roll crimp.Cleaning is easy by running a wet swab through the bore a couple of times followed by a couple of dry patches and a Ballistol patch.
Plastic has no place in Gun Powder loads as the internal temperatures are too much for plastic. I do use cut down Reifenhauser type shells because the tube body is thinner and makes a better roll crimp. Compression formed shells have too thick a body and have a somewhat smaller internal diameter and do not work well with card and fiber wads. No plastic wads pass through my SASS barrels with Gun Powder.
I save those compression formed cases for loading my daughters shells, and they work well with nitro powder, plastic wad and a pie crimp on my ancient MEC machine. The shells I use for Gun Powder loads are trimmed to 2 1/2" and are loaded, shot, and discarded because usually they are burned to a crisp. The endless supply of range pickup empties are my usual source, but I did score a couple of hundred Federal paper 12 gauge hulls which load beautifully and are very period correct.
Yr' Obt' Svt'
Bunk