Chamber Polishing – I advocate doing this in Semi-Auto rifles to give reliable extraction.
The following is copied from a Word doc. Any questions, respond here. I will repeat - if the chamber is undersized, get it fixed (usually with a NATO spec chamber reamer). A cerrosafe cast of the chamber is a good idea.
Some of the good NRA High Power 'smiths do this to ensure reliability and so do I. Chamber polishing is intended to allow a rifle with a well shaped chamber to extract cleanly. Chamber polishing will not fix out of shape chambers, bad extractors, oversized or undersized gas ports, loose carrier keys, small bolt tails, etc. Also, if you have problems with chambering, this is not the fix. If you have other problems, those should all be found and dealt with too.
The big thing to remember about chamber polishing is that we are just trying to take the "meat hooks" off the surface. The surface looks sort of like a mountain range, and the high points are sharp. The brass is pressed onto the sharp points, grab the brass, and make extraction tough. We are just trying to knock the high points off, but leave the valleys. We are not trying to change chamber dimensions in any measurable way at all. The next thing to remember is that you should polish the body and you should not touch the throat. In between the body and the throat, there is little advantage to fussing with it and it has risk that you will mess up the throat, so my advice is to stay away from the neck and throat.
Several methods are cited by folks on these threads.
Some actually recommend JB on a chamber mop turned with an electric or cordless drill. JB is finely divided clay in grease and is well known to be harmless to our pampered bores. It is great for removing copper, carbon and primer ash, and will make the chamber look brighter, but I am skeptical that it will actually take off the high spots in the metal. If your chamber looks better after a scrubbing with JB, it was probably dirty to begin with (which might have been the source of any reliability problems).
Some folks use Flitz or Simichrome on a chamber mop. This method is simple and can work. My concern is that most chamber mops can get to the throat. Polish the throat with a spinning mop and abrasives, and you may mess up your throat. Make sure that the mop cannot get beyond the shoulder and you should be OK with this method. A rough chamber will not be fixed this way, and a chrome lined chamber is unlikely to smooth out this way too, but a good commercial chamber can be made to extract slickly with Flitz.
My method – Thoroughly clean the barrel and chamber and inspect it. A chamber mirror helps a bunch here. Remember what it looks like for later. Mount a dowel on a piece of GI cleaning rod and sand it to the same taper as the chamber, then split it lengthwise to hold a 1" wide strip of abrasive paper. Trim the length so that the tool will enter the entire length of the chamber body, but it won't go into the neck. I use 220 grit wet-or-dry abrasive. Emery cloth WILL cut too easily and enlarge your chamber. If you want a GI chamber, have the appropriate reamer pushed into your chamber – don't use emery cloth to get it. I like 220 wet-or-dry because once the surface is smoothed down it just does not want to cut anymore. Run the tool on a drill at low speed, with oil, and work it back and forth in the chamber a few times, then clean the chamber and inspect it. You are looking for places that are smoothed out and places that are still as rough as before you started. The rough spots will need more work. A chamber mirror is useful. If you still have rough spots, continue. If the paper starts to load up and look dull, replace it. Follow up by cleaning the chamber and barrel thoroughly, inspect it again and test fire it.