I use a Sight Mark Mil-Spec holographic sight on a railed 6" barreled upper. The Mil-Spec is like a junior EOtech 512 and will fit nicely on a railed upper.
Half price of EOtech.
MDG that's not a good one as it isn't made anymore. They call it a reflex sight. There are lots of them like the trijicon RMR, the Sig Romeo0, and the Burris Fastfire.