Soaking is your friend.
If bore solvents don't do it fast enough, use a gun safe carbon remover.
The worst carbon problem I ever had was the gas cylinder and muzzle brake on my AK-74.
I tried everything........ packing them with paper towel and soaking with bore solvent, using a lead away cloth, and Slip 2000 Carbon Killer, nothing worked.
Then just as an experiment I used Shooter's Choice Extra Copper solvent and the paper towel packing.
To my surprise the carbon was removed down to the hard chrome plating.
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