The hulls on the white box Winchester and their Universal line are VERY thin and deform easily. The aluminum base gets wedged into the chamber, making it hard to rack them out. They throw autoloaders fits. I've copied our process for polishing chambers below...
The polishing method commonly used on gritty shotgun chambers:
Using no courser than 800 grit automotive sandpaper, wrap a section into a tube small enough to fit inside the chamber and lightly lube the chamber with light gun oil (Rem-Oil, etc.)
Attach a 12 Gauge bronze bore brush (on one section of rod) to a corded or cordless drill.
Insert the brush into the chamber, let it grab the paper and spin it for about 5-8 seconds, moving it slowly in and out a bit. Wipe the chamber clean. Run a bore snake through the barrel.
Repeat the above with 1000 grit, then 1500 grit automotive papers and light oil in the chamber. Wipe the chamber clean and run a bore snake after each grit.
Cover the bronze brush with a piece of cotton cleaning patch. Wipe a very thin coating of Flitz or Mother's Mag Wheel Polish into the chamber. Spin the patch for 8-10 seconds. Wipe chamber.
With a dry cotton patch on the brush, run it another 8-10 seconds. Once complete, spray the chamber out with gun solvent (Birchwood Casey Gun Scrubber, etc) and follow with a bore snake.
Lube the chamber and bore with a bit of CLP and then run the bore snake through again.
As with anything that you can't easily add material back to, go slow, take your time and don't overdo any of the steps. All we're looking to do is knock down the machining marks that are grabbing onto those textured shells and expanded bases on the way out. We aren't trying to turn it into a 10 Gauge.
William
http://www.OR3GUN.com