I would like to stress t6hat the tap is final solution and must be done with utmost care.. Scratching of the chamber is very possible and hard to remidy. Try the oversized brush inserted from the breech and then pulled backwards. The bristles will fold to allow the passage of the brush into the bore but will then be angled such that moving the brush to the rear they will act against the front edge of the neck. Kroil would be a good first step to loosen any bond between the chamber and the neck piece. Pulling the brush to the rear will be difficult and you may have to resort to a rod from the muzzle and a rubber mallet,, I seriously doubt you'l have much luck trying to pick the part out without scratching the chamber. If you have cerrosafe you could pour a long chamber cast(include a bit of the bore) and the errant piece will come out with the cast..
PS--the best way to determine the tap to use is with a expandable plug gauge, just insert,expand, remove and measure.