I aquired a 686 not to long ago that appeared to have had untold amounts of 38 spl shot threw it & had alot of carbon in the step were the 38 ended & the 357s bottom out in the chamber..
I tried the bronze brush with hoppes#9 + the chore boy on an old 30cal brush but just couldn't get the carbon to come off.. I took the advice of useing spent brass belled out & sharpened the inside case mouth with a tapered reamer & drilled the primer pocket & put a bolt threw with a nut on the outside so I could attache this to my cleaning rod for ease of removal, This realy did the trick as it was chipping the carbon off but still left a small amount behind that I then again was useing the bronze brush with chore boy + hoppes & spun it in a cordless drill on slowest speed & finished with a few seconds in each cylinder hole with the Brownell's Flex hones in medium & then fine..
I was looking down each charge hole with an eye loupe & a bore light..
Here's a pic of the "Tool" I made from a expanded 357 case & made it up to fit my cleaning rod.. Worked well enough that I had to share the tip.. You have to expand the case every 8-10 pushes into the cylinder & being it's brass won't hurt the interior of the cylinders bores..