If Tom is following, possibly he could elaborate? Are the failures separation of the tubes or failure of the dovetail joint?
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=295263&page=6
The only dovetail joint on the M21 barrels is in the lump at the breech. The remainder of the barrels from that point forward are joined with soft solder, as are the ribs. The use of soft solder was necessary because the barrels were made of heat treated steel, and the heat of brazing them would have drawn the temper of the steel. I have often heard of M21 barrels separating, but many other guns have suffered the same problem, for whatever reason.

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