Can't really generalize...Sidelock and Boxlock very different. And each gun has different metallurgy, dimensions etc. However the weak point on a sxs double is the 45 degree line angling back from the corner of the water table and the breech. So sidelock most always stronger than boxlock because there is more metal in this plane. In an O/U, most bending stress is similar location but there's more tension on the cheeks and less bending because the pivot point is usually between the bbls not below them. This is a good point in favor of O/U..but the sidwalls that bear this stress are necessarily quite thin. But the "beam" is also "thicker" (deeper) on an OU than on a sidelock, and it's a U channel not a rectangular bar. I think if you consider the pluses and minuses and do the math, you will find SL strongest, then OU, then boxlock. Again, specific guns may be out of order.