Try spraying it heavy with some Ballistol and let it sit for a while. Hit it with a good brush (optional) should clean right up.
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I did this with a stripped ar bolt, put the pieces in a "solo" cup and sprayed it good with ballistol let it sit overnight. Actually, just rinsed it off with steaming hot water the next day and it was clean, very clean. Not a bit of carbon left on it, the bottom of the cup looked like you drained some well used oil...nasty black mess.