That does sound rough. In hindsight actually you should be probably thankful in your prudent maintenance to find the issue before it discovered you. I am thankful to be reading about a broken gun instead of a cataclysmic failure and a broken you. Hopefully it is a shot scratch but even then it may have weakened barrels which would give me pause. Sounds like a nice gun but also sounds like you have plenty of other big bore guns to safely shoot now that your a small bore man now with that new to you 16 gauge. Out with the old, old and in with the new old.
BTw, if you have a dentist friend go and get an old probe from him, if it is a crack that probe will feel it like a cavity toot sweet. I think the brass gizmo Phil worked up is great but you may need a sharper point. I am sure there are lots of thoughts to a cause and cure. I try and never tell a man what he should do with his time, money or toys and hopefully a modest cure and solution is available, but doing nothing and moving on is an option.
Also just remembered Edgar Spencer is the sites resident and practicing metallurgist. He may have some thoughts as well. A while back we got into a discussion regarding steel re-crystallization and old barrels, but don't think this what is happening. He maybe able to weigh in. Could be a combination of things that are finally rearing its ugly head.