Well of course the first thing it to get the locked up plug out.
For that if its really stuck, I would use the boiling water first then put either PB Blaster or Bolt Buster in with the barrel locked vertical and leave it set 24 hours if it is still stubborn I would heat the out side of the barrel but not long enough to get the plug hot [this expands the out side while the plug stays the same then more penetrating oil and some leverage. I have heard of some frozen so bad that it took radical means to get them loose but thats not normal, I never had one come in to the shop that I did not get loose.
To keep this from happening again. If you are using 777 thats a big part of the problem. Blackhorn for one does not do that. If you are tightening the plug to tight to start with thats part of the problem; most manufacturers recommend finger tight, and if you notice the premium quality guns can be taken down with the fingers [Triumph,Endeavor even one of the better CVA's and I hear an Omega in that class is in the works].
So if you insist on using 777 us Sabotloaders tape method and realize that there are a number of different thickness of tape. I use the white and put a very light coat of synthetic oil [Birchwood Casey] for most of mine even the Triumph and the Endeavor but I have to use pink on the Omega.