Well, Sir, you are getting lessons! First of all, the joint on a Darne barrel (usually has some engraving around it to hide it) is where the barrels are fitted into the monoblock, a solid steel forging that has been machined to allow the barrels to be soldered into it.
Where myself (and others) are having a problem, is your gunsmith saying the barrels are thin at that point. The problem for us is, the steel is typically over .100 thick in that area, of a typical Darne. Plenty of steel. Even with a bit of pitting. Even with some chamber lengthening, backboring, polishing and whatnot.
Any "Gunsmith", even a hack, should have some measuring tools to stick down your barrels PRIOR to cutting anything, which, would have told him all he needed to know PRIOR to turning the tools loose. This sequence of events gives us all great pause.
Is your gun unsafe to shoot? Maybe, but, I and a few others are having a tough time following the logic and series of events to where they are by your description.
Now, more than before, you need the services of an expert. I've given you one, but, there are others, and, sadly, I am at a loss to explain the actions of the "gunsmith" you enlisted. You are not the first guy to find himself in that boat. It is also possible that the gun is quite safe to shoot.
We don't know that, at this point. Do yourself a huge favor. Contact Mr. Merrington, explain what just happened, and do consider letting him examine what is left, and take his advice as to what to do about it. As an aside, I imported Darne shotguns to North America for over a decade, and Kirk was my warranty man here in the states, with factory support when it was needed, which, wasn't often-the guy knows his way around a Darne.
An inspection is cheap.
Good Luck!
Best,
Ted
PS Almost forgot to tell you, it would be prudent, from this point forward, to ignore the postings of one homeless jOe. He'll not add anything you need with his spewings. There is a feature on the site to allow you to avoid his posts, and many here have done just that.
