Brownells Shotgun Wad Solvent does a great job. Great for removable chokes and ported barrels. Just don't let it get near a plastic stock or high gloss finishes.

You may want to try "Gumout" Carb and choke cleaner with a brass brush chucked in a low speed drill. Be very carefull of the Gumout around your none metal surfaces as it can damage some wood and and of course plastic surfaces. You may want to change up your ammo. Some Ammo has better quality componants, but my totally uneducated opinion is Plastic fouling is worse with hot barrels.

A tornado brush in a shotgun with good gun solvent some elbow grease with time and you will get it out.

I use Kano Kroil and a tornado brush. Any good solvent and a brush that breaks up the plastic enough to let the solvent get under the fouling and lift it will work. Plastic fouling tends to trap moisture against the metal so getting it out is important.