Thanks for clarifying barrel flip, but I'm glad you noted that the barrels do rise. Maybe, for the same amount of rise, the s x s will hit a bit lower than the o/u.
Possibly, on a free hanging line on a gun with no cast you may get an even rotational recoil right and left. Maybe, decent, or not so, gun fit will not allow a comparable rotation to the left for a right handed shooter because of different hand grips on the wrist and forearm, and cheek contact in an uneven manner on the stock. I've noticed gun fit can be given quite a bit of importance on affecting point of impact.
Also, I don't believe a flat rib necessarily lines up divergent to the horizontal [lane of the bores. They're on different planes, but I bet it's pretty likely to see the line of the rib converge with the plane the bores are on. Wouldn't you want bores to point a bit up from the aim since the shot may 'flip' down.
I'd still suspect it's more likely to limit muzzle rise on a longer barrel gun, but the longer a shot column is in a barrel, the more it can be affected by any movement. Probably wouldn't matter though as much as on a s x s rifle.
