I had the same sort of thing happen with a 22 rifle. The gunsmith at Duncan's gave me a piece of thin, woven material that looks like a kitchen food scrubber. I put it on a patch tip (the slotted one) on the end of a cleaning rod attached to an electric drill and put it into the chamber. I ran it for about a minute and that fixed the problem. Basically, what I did was to just polish the chamber. The rifle has been fine ever since. You can find the material at grocery or 99-cent stores. It's just a thin, abrasive rug-type material, usually green in color.