Another less extreme method of fixing this problem is to make, or have made a steel or brass disk which is the same thickness as the gap between the current breech plug face and the end of the bore. Actually, if the disk is about .001 thicker than the current space, it will seat out nicely and seal the rear of the bore, like it is supposed to do.
Using this method, the barrel and breech tang will not have to move. As they don't move, relative to one another, the pins, screws and lock will interface with the barrel without modification.
Zonie
Using this method, the barrel and breech tang will not have to move. As they don't move, relative to one another, the pins, screws and lock will interface with the barrel without modification.
Zonie