I purchased an old 870 Express years ago with a stuck choke tube (rusted in place). After I got it out by heating and the judicious application of penetrating oil, I cleaned out the threads on both the choke tube and the barrel. Before I put the choke tube back in, I coated the threads with an automotive anti-seize compound normally used for spark plugs and O2 sensors (appeared to be graphite based grease). I never had problems after that, of course I'd put a fresh dab of compound on everytime I removed or replaced the choke tube.
Steve

Approach life like you do a yellow light - RUN IT! (Gail T.)