I use Hoppes #9 for end of year barrel cleaning. For sabotless I follow with a copper remover, but never have a lot of fouling.
To get a shredded sabot out, I soak it with Kroil and go to town with a brass brush.
Montana Extreme makes a plastic fouling chemical that is supposed to work well, and Brownells sells something along the same lines for shotguns. I haven't ordered any yet.