We may have to get the resident metallurgist Edgar to weigh in, but I think some of the pros have figured out for consistent shooting that sxs have some barrel flex. O/U's I think have it some but its supposedly more manageable.
I have heard about barrel flex before but, I don't know. Here is an article on it, maybe something to it.
https://shotgunreport.com/2016/04/06/barrel-flip/