I would try the PB Rust Blaster too. That is pretty amazing stuff. Just give it time to do its work. One it works for a day or two, when you put your removal tool (screwdriver or whatever) on the breech, give it a couple smart whacks with a mallet. That might be enough to vibrate the rust loose in the threads. And, another tip, once you get it started, continue to spray it with the PB and work it a little at a time, back and forth (or in and out) whatever you prefer.