Some people like to run an "on the fly" variable optic setup, like this Samson 3.5x Magnifier
+ Trijicon MRO
both on Samson's Quick Detach mounting system.
Above photo'd:
CMT Upper | Gemtech 5.56 SBC | Magpul PRO BUIS
| Geissele MK8
rail | Samson 3.5x Magnifier | Trijicon MRO in a Samson Mount