Long Guns: Meprolight FRBS Back-Up Iron Sights
In any case, if you use optical sights on your M4 or other combat carbine, I highly recommend that a set of BUIS (Back-Up Iron Sights) be installed. I use Trijicon, ELCAN, EOTech, and Aimpoint optical sights because I test a lot of carbines and like to stay familiar with the optical sights I’m likely to encounter.