+1 on all you people suggesting Flitz.
I went and tried it again with a bit more commitment. I diluted the Flitz a little just to be extra careful against using anything too abrasive. Just used some Q-tips and then wiped away with a clean soft rag.
The flash burn rings started to rub off. Attached is the chamber face of my Ruger GP100. The left side still has the ugly stains, and the right side of the cylinder which has had the carbonized stains removed.
Then some Break Free for a final cleaning and a touch of gun oil.
I'll also try those Kleenbore Lead Away wipes when they arrive.
Well, no excuse now to have an ugly revolver!