For a proper comparison test, make sure you clean it thoroughly before you shoot the new breech plug. You could have one of those rifles that don't shoot worth a damn till you have a good bit of fouling. From your latest post, it kind of sounds to me that you haven't been getting your rifle clean. That black stuff wasn't plastic fouling. Plastic fouling doesn't come off looking like you described. It comes out as little pieces and thin tendrils. What you pulled was carbon and soot. As sabotloader pointed out, the sabots of today are many times better than they used to be. Years ago I would end up with enough plastic in my rifling to mold a new sabot after around 30 shots. With todays higher end sabots from MMP and Harvester you won't be getting any plastic fouling unless your bore is ragged and needs some J-B run through it or you are really cooking it hard. Only one of my BP rifles will shoot sabots worth a damn so my experience with them is limited but I do know that you definitely don't shoot/load them on a hot barrel no matter how high end your sabots are.