I have had plastic build up over time in my Knight (several sessions at the range) but never during a single session.
The powerbelts will probably help some, but they can add both some plastic AND lead, if you don't shoot the plated ones.
It sounds to me like your bore is a LOT rougher than it should be. If I was you, I would try to smooth that bore out some in one of two ways:
1) shoot a couple dozen patched round balls or leads conicals thru it
or
2) Lap it some with J-Bs (or other brand) bore paste. Work at least 6 patches of the stuff down your bore in quick, short pumping strokes....... working a clean patch the same way between each coated patch. Then clean all the residue out of the bore with your normal cleaner.
Either of these methods will reduce your build up and increase your accuracy.
BTW, I did the round ball thing first with my Knight after I had shot sabots only for a year - accuracy was noticeably improved. And I use the paste once or twice a year, whenever the gun's accuracy seem to tail of a little - works like a charm.
