While Lab's recco on proper lubrication should be taken to heart by all double gun enthusiasts, I believe there is another culprit at work here: Inertia.
Large bore guns generally fire large charges of shot and powder in heavy massive barrels. When things go boom, the laws of physics take over. And the whole mass, barrels included, want to go forward. In an 8 or 10 bore gun this is a relatively LOT of metal... all held back by a narrow little hook and some relatively soft metal.
The workmanship and solidity of the lump attachment is yet another strength issue.
Fire enough Jupiter-size shot and powder charges through a double gun like this, and things will naturally want to come apart.
On the other hand, I believe the sheer weight of the barrels and their "hanging weight" on the hinge pin has little to do with it.
just my opinion...
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