Well, there is no way any one can know from afar what your issue is, however, you wrote you have never 'drilled' your plug. If you have never used a 1/8" drill to remove carbon from the flame channel of your breech plug, it is no wonder you have blow back. It is possible if you use a 1/8" drill in the flame channel, and remove the carbon build, you will have no blow back anymore. All you need do is spin the drill by hand, but if the carbon is so thick, you may need to start with a 1/16" drill first.