Mr. Powdergun:
While I personally have not had to deal with this issue, it is my understanding that this is quite common issue for those that use sabots on a regular basis.
It can also be an issue for competative trap shooters (plastic wad fouling).
The appicable fix here in both situations is to swab with hopes #9 or shooters choice to get out the palstic fouling.
So, I suggest you get a good bronze brush (as this will be easy on you barrel) and use one of the solvets specially formulated to deal with shotgun wad or plastic fouling.
The solvent should be strong enough to cut the fouling with out alot of scrubing.
I have seen this "fix" used on more than one BP board I frequent.
Regards and all the best
R