The FNS is not a platform that you'd want to insert an OEM 9mm barrel in place of the SW40 barrel, in an FNs40 upper. The outside diameter of the SW40 barrel, which is relative in size to the opening in the upper slide, is considerably larger diameter than the 9. There would be a lot of 'slop'.
The 40-9 conversion barrel accounts for the differences in tolerances/fitment, which translates to safer and more reliable function.
I guess that would make sense if the barrels OD are that big of a difference.
You mentioned fns I'm assuming that's the same for the fnx though..
If it is so you know how much those barrels run new and who sells them? And another question is how much in comparison do they sell used for? How often do they cone up on the classifieds here?