Cut a piece of 1/4" wooden dowel rod about 2-3 inches long and cut a slit in one end with a hacksaw. Then cut a 3/4" strip of #600 emery paper and put one end through the slot, wrapping the paper around the dowel, emery side out until it will just go in the chamber. Put the dowel rod in a drill and run it slowly for 10 seconds or so.
It may take 2 passes but shouldn't take more.
(Maybe needless to say, but remove the barrel and work from the chamber end; one guy asked if what I recommended wouldn't hurt the rifling when he pushed the rod through the barrel to the chamber.)
Jim
