Just this week I used a 12 ga. chamber Flex Hone to polish my 870 SuperMag's chamber. It had severe extraction issues and the chamber was rough as a cob to the view and touch. A generous poster in another forum sent it to me and it has been returned to him. There are short videos on YouTube that demonstrate technique. I used the specified lubricant for the hone. It took 1.5 minutes of use to smooth out the chamber. Brownwells has them. The Flex Hone consists of numerous abrasive pads on the end of wires that compress allowing entry and the rotation of the drill causes them to push against the walls.
https://www.youtube.com/watch?v=RzAm5Ni0QCw
