If I were going to remove the breech, I would place the barrel in a sturdy vice with leather (to protect the finish).
Keep your work as close to the vice as possible (no more than two inches sticking out). This will reduce any flex in the barrel while trying to break the breech loose.
If it has a snell, place some leather around that area or some thin lead to prevent marring, and a 2 foot crecent or properly fitted pipe-wrench (opposit of snell, and careful not to let slip).
Apply some pressure at the tip of handle and rap the handle with a mallet or about a 2# shop hammer.That should break the torque or the breech.
If it has a tang, I would make sort of a barrel wrench out of some 1/2 or 3/4" sq stock.
Make one about 18" long and the other 3".
Clamp the 3" piece on top of the 18" at the end and drill (2) 3/8 hole 1/2" from the end of the 3"piece. You can drill all the way through both pieces for a bolt and nut, or drill and tap one side and clearance for the other.
The 3/4 is prefered because of the 3/8 hole.
You could use 1/2" stock and a 1/4" bolt, but I would use a grade 8 bolt if you have to go this rought.
Before I get scolded, the wrench can be placed on either side of the barrel flats and tightened.
Don't forget to use sometning between the barrel and the wrench to protect the finish.
Hope this helps, and good luck with your project!
If there is a will, there is a way!!!!
Ronnie, from the hills of Tennessee,