Here is a solution I haven't seen for some years. It's dangerous to the point of being truly frightening, and probably illegal in some areas.
Simply plug one end of the barrel and fill barrel with mercury. Wait overnight. CAREFULLY remove mercury and CAREFULLY brush out residue from barrel. Dispose of all materials properly.
Mercury is extremely active when it comes to most metals. It will absorb* most metals, especially soft metals like copper, lead, etc. I don't know what it'll do to chrome lined barrels though. But this was at one time a solution for heavily fouled barrels.

*The refining of many metals, i.e. gold, silver, etc was accomplished by pouring mercury (aka quicksilver) over the crushed metal ore and then boiling it off. The residue after boiling was the refined metal! The residue also included scrambled brains... that's why there is an enduring icon of the old West, the lone prospector, the crazy old coot with mining pan and pickaxe, mumbling to himself as he crossed the desert with his mule. The point is:

MERCURY IS EXTREMELY TOXIC! MERCURY IS EXTREMELY TOXIC! MERCURY IS EXTREMELY TOXIC!his