Interesting topic! I can only offer this.

All SKB, Miroku, Browning and most other modern production SxS use a double underlug, no extra bolting. Never have seen one get loose from shooting modern full pressure ammo. A few damaged by lack of cleaning/lubrication or stupidity. Merkel uses a cross bolt and the double underlug, the crossbolt is probably not needed. The most basic lock up of modern guns may be the Stevens/Savage 311 and variants. These guns get good mileage.

I would also like to see high speed pics of just exactly what happens when the gun is fired.

Chief