I own the AR556 model # 8500. I have added the Magpul SL stock and hand guard. I also installed a ToolCraft BCG with a BCM extractor upgrade spring kit. And, I had a Magpul rear MBUS sight in a box for another AR build. I decided to add that too. Now I can aim thru 2 apature holes. Last but not least, I added a AIMPOINT red dot. I paid $449 with FREE shipping 2 years ago from GRAB A GUN. It's a real tack driver!
You will really like it!