myers1361,
You have failed to explain your cleaning method. Do you use a brush? I had a barrel that shot lights out when new and fell off really bad as the round count went up. I, at the time, subscribed to the old notion of not over cleaning a rimfire barrel. Yes, I cleaned it, but not the way I clean now.
I scrubbed the crap out of it till I got nice clean patches. Brushing, with a good solvent, patches, brushing again, more patches. Took awhile. After clean patches, the rifle shot just as good as it did when new after a few fouling shots. I now clean different than I used to and have no issues. I shoot competitively and clean after every card, as do most all of those I shoot with. I do not remember seeing anyone using Kroil. I do use Kroil to soak a center fire bore over night before cleaning with an appropriate solvent, but not on rimfire.
Since your rifle is used and older, I would do this. I would clean the bore to a clean patch using a good solvent and a brush with a quality bore guide installed. IMO a good solvent would be Bore Tech Rimfire Formula or Pro Shot Products. If the rifle will not group better after that, I would find someone with a bore scope (preferably a qualified smith) and have the barrel checked. This is just how I would proceed. Good luck with your rifle.