Since you already have a piccatinney rail, I would suggest this:

Burris AR-332 Prism Sight

They also have a 3x version, but similar $$$.

Link to Midwayusa:

Burris AR-536 5x 36mm Prism Sight Ballistic CQ Reticle Matte

And the lesser one::

Burris AR-332 3x 32mm Prism Sight Ballistic CQ Reticle Matte

Read the reviews....

Regards,