Not so for the vast majority of stacked double rifles. The Merkel and all German and Austrian O/U double rifles have a conventional hinge pin and various fasteners. The same is true of the Browning's and their knockoffs.

Here's a look at my Merkel 323E. You see the normal hinge pin. It also has Purdey bolts and Kirsten top fasteners. I would opine this is a pretty darn strong arrangement.

Curl