Does it look dull or shiny? It may not be a carbon ring, but carbon in the haze cracking at the end of the chamber.
I use a nylon brush a little bigger than normal, a cal larger, plenty of carb cleaner and spin it up fast in my drill. Alternating between carb cleaner and CLR.
You need a carbon solvent, brake clean, carb clean and CLR all work extremely well.
DO NOT GET ANY OF THEM ON YOUR STOCK FINISHES, BLUEING OR PLASTIC.
Cheers.
P.S.
Expect to replace your nylon brush after this, it will melt.