I would be inclined to ream out the touch hole to about 1/2" diameter. That might lower the stress enough for it to be only mildly unsafe.
In Ancient Times, e.g. the 1980's the late John Baird got me in contact with various people involved in gun failures. The first thing I looked at was a brass barreled blunderbuss made in India, and sold by Navy Arms.
Val Forgett had been unaware of just how they made it.
Every now & again some Righteous Person says we cannot consider an entire nation bad because of some junk. I in my terribly bigoted opinion, perhaps enhanced by experience, differ. I might suggest no one ever shoot a muzzle loading gun made in India.
Well, some fine fellow had been shooting blanks through this blunderbuss.
As he seemed unaware that even without ball one can get rather high pressures in a gun, he loaded it with a 12 ga shell full of black powder. Pushed down a load of newspaper for a wad.
The breech-plug flew past (fortunately) his head.
I was puzzled, as I could easily screw that plug back in place. It was very, very loose. The fine male threads were so small, and the female threads so large, that only the very tips of the threads were smeared, just a little, as the breech plug blew out.
Had the male & female threads been sized to make a decent fit the thing might have stood up to his blanks longer than he could.
One other point which might be ignored now & again is that threaded connections in brass should be with COARSE threads, and not fine. This is, or was, well known in various mechanical circles. I doubt that I could come up with a reference now.
