I've had extremely good results with Slip 2000 Carbon Killer but I've run into a situation where it can't help.

The problem is a build up of crusty fouling in an AK-74 hard chrome lined Bulgarian zig-zag muzzle brake.
After soaking and brushing extensively there's a ring of crusty fouling in the front of the expansion chamber that just won't come off. The rest of the chamber and brake are back to bare hard chrome.

I emailed Slip 2000 for advice and they replied that it's probably fouling that wasn't removed and has hardened over time.
They said that Carbon Killer can't remove this.

Before I found Carbon Killer I was cleaning the brake by packing it full of paper towel, soaking the towel with Hoppe's #9 and letting it soak for hours. After soaking, I'd remove the heavily stained towel and re-pack and soak more. I usually did this for a full 24 hours until I stopped seeing copper fouling left by the plated steel bullets.
The expansion chamber remained black with carbon that just wouldn't come off even after scrubbing with a brass brush.
The first time I used Carbon Killer the expansion chamber was cleaned down to the hard chrome, but this ring of build up won't come out. The build up appears to be small lumps of material with a very faint coppery color.

I want this OUT.....Any other cleaning product or method to try?

There is also a Kano Labs (makers of 'Kroil') product used for dissolving carbon in engines called 'Kreen'. This stuff is extremely aggressive but won't hurt metal or plating. Kano Labs 'KREEN'. Another angle is the oldest treatment in carbon removal, 'hot-tanking'. Take any solvent, put the parts in it and heat it up. When combined with ultrasonic vibrations, there isn't any build-up I know of that will withstand it. You could try boiling the brake in water with a small amount of dish soap or stronger detergent. I know any brake with an expnsion chamber at the muzzle, like an AR A2 FH, is subject to this kind of build-up, and it can be a bear to get out.