Trijicon makes an ACOG the army uses from time to time with a fiber optic red chevron reticle, and a redimentary iron sight on top. Works ok, nothing precision, but can be used effectively with practice. It's for if the optic gets damaged or the glass fogs over.
in the realm of no mag sights, when they take a dump, you just flip up your Back Up Iron Sights... Hence the na,e.