Hey Musclecar - I just wanted to come back to this problem of yours. Were you able to get some C-4 or similar Carbon solvent? And do you have or know someone who does have a bore scope? That will show you exactly what's going on inside the bore/chamber. I found a couple threads about carbon rings by searching i-net, too. There are differing views on what/how it builds up, and how to remove it. Here's one - htt p://www.rimfireaccuracy.com/Forums/showthread.php/9488-Carbon-Ring!!
As I said, I use the C-4 letting a wet patch soak for overnite or at least a few hours. An alternative would be to use something like Remington 40-X Bore Cleaner or JB Bore paste on a tight patch, and spun in the chamber on a drill. I did that before I got onto C-4. Using a bore paste pushed thru the barrel (Not Drill) can smooth the barrel similar to lapping. Here's a thread to that effect - see Post #20 - http s://benchrest.com/showthread.php?97313-Rimfire-Cleaning-Is-Brushing-Bore-Absolutely-Necessary/page2& - If you read the whole thread you'll see the different opinions on this, but I think it's a 'real problem' and proper cleaning can solve it. Just using a brass brush and #9 may not be the answer.
Hope some of this can help you,
Ed