I like PB Blaster for anything rusted tight.....works when even WD40 cant. If you are worried about the finish....I'd hang the barrel with the choke down....and spray PB blaster into the barrel. Put a pan under it to let it drip. I'd repeat that a few times over a couple of hours...(or more) ...then carefully try and loosen the choke.
If that does not cut it....you could try to gently heat the barrel with a hair dryer and cool the choke with some "air in a can"....the kind you use to clean computers with.....that will cause the barrel to get larger while the choke contracts....hopefully separating the two long enough to remove the choke.
-of course this would only work on an internal choke...not one of the ones that thread onto the outside of the barrel.
-just my guess though....good luck!:thumbsup:
_alternately..you could just oil the heck out of it, shoot it until it gets good and hot and try again.