I'll share my experience on suspected sabot buildup. I've fired thousands of rounds using sabots through a single rifle, to which there's never been a single shred of evidence that the sabot left residue of any kind in the barrel. The ONLY thing that affected the constant and consistent accuracy of the rifle itself, was the breech plug and flash hole enlarging.
History: Back in IIRC "97" when the Encore hit the market, I "thought" the same thing about sabots. When my groups started suffering, the first thing I thought about was "plastic". So what was the first thing I thought of to do, get the plastic out of the barrel. I won't go into the different processes I tried, but NOTHING I tried improved my groups, in fact they continued to get worse.
So I made the call to the most knowledgeable man in N.A. on Encore rifles. His first question was, "Have you replaced the BP?" Groups instantly went back to normal.
This isn't always the case with different rifles or shooters. Its just one major contributing factor long time shooters have recognized.