Kroil would also be my suggestion. Try soaking the end of the barrel in a jar of it overnight.
If not, then heat would be the answer, or there is a tool made to take care of such a thing.
Be sure to clean the threads out. I would use a 10ga brush on a section of cheap 3pc cleaning rod. Chuck it in a drill, and use oil.
I would suggest using choke lube, and threading the choke in finger tight. Even flush choke tubes. Never had a stuck choke, and have used this method since I began shooting.
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