If you are referencing normal carbon residue, BoreTech C-4 is the best I have ever used. For hard, baked on carbon from high burn temperature powders, it is a much different situation. If the layer is relatively thin, I've been able to remove it with Ed's Red formula (available from Brownells), scrubbing with bronze brushes and monitoring with a borescope.So I read something the other day. A gunsmith recommending the use of CLR too remove carbon from rifle barrels and muzzle breaks. Anyone else hear of this practice?