Several years ago I acquired a old set of 12 bore muzzle loading barrels from Belgium with the intent of building a shotgun. Long story short, I proofed the barrels and thought all was well. On thoroughly cleaning the barrels, I discovered that the things felt quite loose as my cleaning jag and patch reached the breech area. Further checking made clear that the breech area was indeed enlarged but I had no way of determining to what extent. Rust? Burn out? Project shelved. Moral of the story for me was to be sure that I checked things thoroughly. Sometime I'll tell of you about a muzzle loading rifle barrel I acquired with hopes for another project.
J.B.