With break open guns, the issue is keeping the barrels on face. It's as much a matter of geometry as it is mass of metal. Break open guns try to open when firing due to the push back on the breech face and the barrels wanting to flip down from the bullet travel. Barrel wall thickess is enough to keep the side expansion in check, or the gun wouldn't last but a couple of shots. The pivot point on an O/U offers an advantage as it takes some of the leverage away from the barrels wanting to flip down, especially for the bottom barrel.

That being said, the hunge points for an O/U are just studs that don't span the width of the action. The walls want to flex away from the barrels, and since they aren't usually very thick, I can only speculate that is why you don't see O/Us in the really big calibers.

Throw in a third fastener, longer action bars ( reduces leverage on the hinge pin) and more support for the hunge..... I think the nod goes to the SXS.

Actually, my opinon of what would make the ultimate action for strength...a snap action ( side lever for my taste) trigger plate lock ( the lever cocks the hammers, the lockwork behind the action body so the bars remain solid) clamshell with Bissel rising third bite. That would take some serious punishment......hmmmmm