Unfortunately, things like this happen on a fairly regular basis with the lesser priced O/U shotguns.
There are many reputable shotgun gunsmiths that can fix this, and make the repair many times stronger than what it came from the factory, and enough so that you will NEVER have that issue with that gun again.
As far as the cause of the failure, looking at the pictures, this was not ammo related. The contact area between the barrels and sleeve was not properly cleaned, fluxed and soldered. The solder itself could have been of insufficient strength, and the reason for the separation.
There are many reputable shotgun gunsmiths that can fix this, and make the repair many times stronger than what it came from the factory, and enough so that you will NEVER have that issue with that gun again.
As far as the cause of the failure, looking at the pictures, this was not ammo related. The contact area between the barrels and sleeve was not properly cleaned, fluxed and soldered. The solder itself could have been of insufficient strength, and the reason for the separation.