If there's no finish to damage or worry about, you can probably get it off with steel wool, then clean and oil it real good. If you can't get to all of the rust, there's a chemical called Ospho (phosphoric acid) that you can get at Ace Hardware that is a rust converter. It changes rust from Iron Oxide to Iron Phosphate, and stops it cold.
It will turn the rust black, however. And don't get it on any blued surfaces! Bluing is actually a form of rust, and the phosphoric acid will strip it off. People have ruined guns by putting Naval Jelly on them to get rust off, it takes the bluing completely off.
If you have a Bluing Kit, those always come with a blue/Rust remover. Birchwood
I have used it on basket cases that needed touching up anyway. Like Ron said though, rust removers will also remove bluing.
Otherwise I like steel wool as well.
"It is useless for the sheep
to pass resolutions in favour of
vegetarianism" ~ R. W. Inge
Andrew have your ever heard on Flinz or Semi Crome ? This stuff would be a good treatment to put on the bolt to perty it up a lot.
If you had a small wire wheel brush to chuck into a hand drill or better a drill press you can really do a good job on that bolt.
I bought a 3 piece el cheapo set at Big Lots and I would imagine Wal Mart has them in the tool section.
Some auto parts stores have a cheapo tool bin with the same 3 ps. set of round wire brush wheels that have the arbor permenantly made on to them, the wire is fine and brass plated I think.
Where eye protection when useing because some of the wires will fly off.