I've sucessfully used Naval Jelly rust remover to clean off the firing discolouration from stainless cylinder faces.
Just dad on some with a cleaning patch or a Q-tip cotton swab, let it set for a minute, and wipe off. Rinse with water to remove all residue. It will look like new!
Be advised that Naval jelly removes rust and bluing too, so it should only be used on stainless steel guns.