not familiar with a 5.57x28 although there are several googles seemingly using that term interchangeably with the FN cartridge 5.7x28..
as mentioned above, the 5.7x28 pistol is marketed as the FivesevenN emphasizing their initials in the caliber name spelled out..
as far as the applicability of the 5.7x28, the FN PS90/P90 [FA selectable] bullpup carbine and the FN FiveseveN USG/MKII polymer pistol are probably the most prevalent..
I have read about some AR applications although I have never actually seen one or fired one in person to rate its usefulness..
the 5.7x28 [.224"] cartridge was developed for specific military purposes and doesn't seem to have migrated much beyond the two basic FN platforms..
hope that sheds a little light on the subject, cheers : )