Gun cleaning solvent and a brass brush or bronze wool. I don't go nuts trying to clean the burn marks off, just the powder/lead deposits.
You can scrub 'til you're blue in the face to get all the burn marks off, but there will just be fresh marks next time you fire it, so I don't see the point in putting in that much effort. The burn marks don't hurt anything or have a negative effect on function. But then I'm not much of an OCD type....