Lot of guys reading the problem of hard cooked on carbon in bores can not relate.
I have all of the products mentioned in this thread. For hard cooked on carbon, I will insert a cork in the muzzle of the barrel, stand the barrel up, and fill with a SUper penetrating oil called Free All. I let it soak for two weeks. We have found that Free All has no equal in this application and in breaking loose rusted on bolts.
Then I drain the oil out of the barrel, and brush with a good bronze bristle brush, dry the bore out, examine with a Hawkeye bore scope. Repeat as necessary.
I recently dealt with a 308 Palma barrel that I bought off the internet for $100 that was black from one end to the other. No scrubbing with JB, good new bronze bristle brushes would remove the carbon. It took three, two week soakings with brushings in between.
I have all of bore tec products, Kg products, and Wipe Out products, they will not touch hard cooked on carbon.
Brownells sells a silicone carbide paste for cleaning heavy carbon fouled bores, 600 and 800 grit. A lot of guys shooting very large calibers use the Silicone Carbide bore pastes to clean up their bores.
Some powders are rough on barrels, check the heat index of your favorite powders!