If your barrel is not cammoed fill a mason jar with Kerosene and soak the barrel choke down for a day or two and then use a choke wrench to unscrew it. If that does not work try Rustbuster then a gunsmith.

 

The one penetrant I've seen work when nothing else has is KROIL.
It's good stuff, and I buy it by the gallon.....

 

Also try heat and cold to break the rust in the threads, a torch or good heat gun will go a long ways to help you free it up then hit it with some cold spray on the inside of the choke with something like canned air held upside down or Locktite has an aerosol that does the same thing to remove rusted bolts.

Kurt

 

I have seen what hat flats said work.