Varying degrees of light will diffract over a round peep. A Square one will throw it off in all 4 directions depending on light levels. A crosshair has been the answer for me. I don't cae about the crosshair and I wish the round didn't diffract as it's easer to line them up on my round pin gaurd for an extra aiming reference but these Nitehawk sites give 4 right angles and the central squared sight picture is free from diffraction in any lighting situation. It's the thrid from the right onthe top. Only pic I could find.
They come in varying sizes as well.