Had a chance to shoot today, but conditions were not good (cold & very windy) so I just shot with the new breech plug installed and the bore had been cleaned with the cleaner I bought. Groups (at 100 yards) were reduced from slightly over 4" to around 2" today.
Plenty good enough to give me some confidence for the upcoming weekend gun season (Dec. 16-17).
When we get better weather I will try the old breech plug and see if groups open back up.