Michael McIntosh has a chapter entitled "Lore: Barrel Flip" in his book More Shotguns and Shooting. In it he talks about how the dynamic forces of the shot create barrel flexing and harmonic vibration, which is influenced by barrel orientation (i.e. OU vs SxS) and barrel length (long barrels flex more than short barrels and have a longer degree of arc from the chamber). He notes that on a SxS the center of the shot swarm will angle low right from the start and the distance will grow "geometrically with distance." This can be mitigated by a properly fitted stock. OU's are more rigid vertically and "they aren't as affected by harmonic vibration." Thus, according to MM's chapter (with a reference to Burrard), factors that influences the amount of barrel flex and harmonic vibration will also influence the the amount of barrel flip. Ribs on a SxS certainly contribute to the overall rigidity of the barrel assembly, which will influence barrel flip and the associated point of impact.
