Not sure what to tell you about that. I shoot a Knight and shoot Knight sabots w/pyrodex, and I have never had a problem with plastic fouling. When I shoot my gun at the range, I usually spit on a patch and run it through every 3 shots or so. I' ve shot it as many as a dozen times without cleaning, and never any problems, but I usually swab it out as it makes it easier to load. Accuracy is always the same...dead on.

Maybe it is the sabots you are shooting?? Maybe the powder??? Maybe some sort of mysterious combo of those and the heat??? Who knows, but whatever it is I hope you get it figured out. That would drive me crazy.