I was getting so frustrated with my groups. Blamed everything from parallax and diopter, to barrel bands and scope mounts... When tyring to line up my second shot my eye blurs the horizontal line and turns my reticle into a diamond. I came the the revelation that I have astigmatism.
I verified this by digging out some red dots that I haven't looked at in 5+ years, sure enough starburst and splotched reticle at higher lumin levels...
I'm due for an eye exam last one was over 25 years ago, it was more surprising because I have better then 20-20 vision.