On my M4 I have a BUIS. However, I primarily utilize an EOTech 516.a65 HWS. “Holographic Weapon Sight” plus a EOTech G33 STS “Switch to Side” On the top picatinny rail. I absolutely love how you can engage targets at close quarters with the HWS & Then switch the G33 over to magnify targets out to 250+ yards. Although, I would like to upgrade to the EOTech EXPS3, XPS2-2. I don’t know if it’s necessarily worth the money to upgrade from the 516.A65.