Amsoil powerfoam, plug bore and foam it up. By far the best stuff i've found. Its made to remove carbon from engines.
I thought I was the only one to use powerfoam on a gun.
It isn't cheap but if you're a mechanic and have a can around (I buy it by the case) it works great.
It will also strip the plastic fowling left from wads in a shotgun barrel better than anything I have tried.