This has been an issue with my 627 as well. Never ever had stubborn carbon 'spots' on my cylinder till this gun. Haven't found the best, simple solution yet, but I have been successful with using a Q-tip and using some Hoppe's, Carbon Remover, Ballistol, and Kroil (all the above). Slow and patient, dab with the Q-Tip...go do some reloading, come back, gently fingernail area...dab again (one solution at a time), and eventually they dissolve. Do not rub with any sort of instrument or abrasive or you'll polish the area...remove the matte. Not good. Got to be a better way. I also have tried to 'treat' the area prior to shooting to prevent buildup, but nothing has worked. I think this is an issue with all the matte finished guns. I'm guessing that if you remove the cylinder from the yoke and submerged/soaked in a carbon cleaner that would be the easiest. I just don't take the cylinder off all that often. Hope we all find a fix for this issue!