Rob;
I am not familiar enough with this model shotgun to say for certain just what alloy beretta used to attach the side ribs. I would find it extremely unlikely that an alloy was used which would create a heat problem. A modern alloy steel would have enough strength even in its totally annealed condition not to burst in a normal shotgun barrel.
What could well have occurred rather than an internal seam in the barrel is it could have been cut into creating a weak place in striking up the barrels after the ribs were attached.
This could have even withstood finial proof & not let go until stretched beyond their ultimate tensile strength due to the ring bulge. The combination of the bulge along with the longitudinal split says to me that two factors were involved. In fact the lower barrel may have only bulged similar to the top barrel had the 2nd factor not been present.
That's my take for whatever it's worth, I understand Wonko will try to make me look like some kind of Idiot, but I'm used to his rants & could care less. I can't really recall him ever adding anything truly useful to this board, always just "Putdowns"


Miller/TN
I Didn't Say Everything I Said, Yogi Berra