The problem in fitting aftermarket sights on to a 416bis that they are a different height then regular m4 sights. (they are lowwer) H&K offers some flip up sights but its odd because you either have to use their front flip up with the rear diopter OR a different hight front flip up with the flip up rear they offer.
Sry if thats confusing. Check HKparts.net and look at iron sight and youll see what i mean about needing to use a certian height sight with certian rear sights.
Edit. 28mm front sight for use with fixed rear sight
33mm front for use with flip up sight
You can also use troy micro sights (front and rear)
keeping in mind it may change the height of the sight picture
http://www.hkparts.net/shop/pc/HK-416-Sights-c60.htm