I saw this posted on the Primo forum, but no answer yet. I cook them on the egg and don't have an issue with the ring or the bark. Perhaps its the 325. Did you try 225-250?
I did have a ring issues on my drums once. Interesting.
I would not put Kingsford in the Primo. The ash will clog up the vents.
Large Big Green Egg
Weber Kettle Blue 22
Weber Smokey Joe
Weber Gold Genesis (Sadly, this did not come with a long keyboard solo)
1 ~ 3-year old Princess
1 ~ 1-year old Caveman