The muzzle end of the barrel or the chamber end of the barrel? If it's at the chamber, it's probably just a crud ring from the powder. If it's at the muzzle end, you can just use any centerfire solvent to get it out, as they dissolve the plastic.
Hoppes makes (or used to) a solvent for shotgun barrels to get plastic wad fouling out, or you can use acetone, but make sure that you don't get any acetone on any part of your stock or forearms, as it will ruin the finish. Also, don't let it sit in the bore for a long time, as it could possibly etch the steel.
I use Remington Bore Cleaner on all of my guns. It's an oil based cleaner with a very mild vegetable matter abrasive in it. It works great on the plastic fouling from sabots and such, just need a little effort with it. It will also help to smooth any imperfections in the barrel, making it foul less.
