Soak with Kroil or PB Blaster for several days, if that doesn't work, put heat on the barrel, not the choke. Be careful and don't get the heat too close to the finish, heat it up from a distance. If the choke is not extended, use a "easy out" spiral thread extractor. I had to get one out for a friend that the choke had rusted stuck. I ended up using a torch to heat the barrel. I was careful and didn't even burn the camo coating on the barrel. Again be careful if you have to use heat. Good luck.