The thing about a good sabot, is that is pushes the debris from the last shot, back down the barrel, every time you load it. The only build up you get is from the powder residue left deep in the riflings, that the sabot doesn't push.
If you have plastic showing up in your barrel, it is time to try another brand of sabot. I would suggest MMP sabots. I have shot a lot of different powder rifles, and a lot of them were inlines, and I have never had any plastic build up in any of my rifles. I shoot a Savage ML II now, and have never had to worry about plastic building up in a barrel. I shoot at least 100 rounds a month through my muzzle loaders, and have for the last 20 years. My powder and bullet supplies last year was a little over $1100.00 and not one cent was spent for any cleaning product for plastic.