Soak the bolt in regular solvent for an hour or so. Then scrub with a brass brush.
As daft as it sounds, E-Z Off works like hot dam. Carbon removal being its purpose. Keep it away from synthetic stock parts though. And do not use in on a painted surface you care about.
Carb or brake cleaner will be less, um, dramatic. They're made to remove carbon too. Mind you, they probably lift paint as well.