I remember someone discussing this problem some years ago, I think it was Bill McGraw but I'm not sure. Sorry I don't remember their cleaning method either but I know it wasn't easy to remove (no news to you). Possibly he used J-B Bore Compound on a tight patch and lots of scrubbing.
I looked up Bill McGraw's articles in the FS but my table of contents only goes back to 2012. He did mention in FS #216 that " ...when I discovered WW231 and AA#2, most of the previous fouling from flake powders (Red Dot, Green Dot and Unique) was eliminated between targets and 25-50 scoring shots plus plenty of sighters could be shot and my bores would be spotless of alloy fouling, lube or even powder fouling. I used appropriate powder charges for 1500 f.p.s. that would maintain headspace for the cartridge cases."
I believe he was shooting mostly 30-06, a bit larger than your .223, but I know you're shooting in the same velocity range and with a slightly faster pistol powder. Once you get it clean, this might help.
Added: Reading old FS articles someone mentioned Blue Wonder bore cleaner as good for getting carbon out. Your "shiny black spot" is likely carbon.