Bob, all good advice above. BUT, the reason it's so hard to remove is that the plug is a jam fit. Grn. Mtn. barrels, for example, don't cut full threads to the base. Don't ask me why. Tough steel, great job on the bore, won more records at Friendship than any other (price and dirt common}, but they still have to be good. And they can't manage this simple thing.
I have taps in all the common sizes I've ground to full threads. Even if you manage to get the @!*% thing out, it will be just as hard the next time. With full threads , it should release easily. I've often "rolled" expensive barrels a flat or two simply by cutting the threads to full depth. IE, they're tight but don't really bottom.
I always retap a breech and see what happens. It actually makes the building process easier and the product much better.