https://www.kgcoatings.com/index.php?_a=viewProd&productId=5&review=write
I have my own Hawkeye Borescope, and I've tried all the magic elixers out there, more than likely. GM TEC (2nd best, IMO), MPro, every product Montana XTreme puts out, etc.., etc.
Nothing else comes close to the KG1 product. (Possibly the "old" GM TEC did, before they "Kali"'ed up the ingredients).
Case in point....the brakes on my pdog rigs have a pretty tough, nearly impossible to remove, carbon build up after I've put 400 rounds through one. I'd finally gotten to where I didn't even bother trying to clean them any more. After I got my first bottle of KG1, just for kicks, I applied a drop or two to a couple of the worst spots of build up, and let it sit for about 20 minutes. I easily scraped the stuff off with my thumbnail.
While carbon buildup on the outside of a muzzlebrake doesn't exactly replicate the buildup inside the bore with it's associated heat and pressure, I can tell you with certainty that this stuff WORKS! I wet a nylon bore brush with KG1, run it two passes through the bore, knock out the crud with a dry patch, then run two KG1 soaked patches through it and let it sit for about 20-30 minutes. Dry patch with a couple patches, and you're now ready to attack the copper.
FWIW, while KG1 isn't advertised as a copper remover, it does a nice job on mild copper fouling, such as that associated with a well maintained big game rifle, etc.
I have a basement full of all kinds of cleaning products, but the only ones I even bother with nowadays are KG1, KG2, WipeOut, Kroil, and Brakleen. If you can't do a good, relatively quick job with these products, it ain't gonna get clean no matter what you use.
Mike