Welcome to the wonderful world of Holographic Sights. I'm not a big fan of these things, but in spite of that my wife wanted one on her Bushmaster Carbon 15 Model 21S Pistol. It's the one she's holding in my avatar. To be honest, it was a bitch to get it sighted in. It was walking all over. It wasn't the gun, it was me. I finally learned to position myself in the same place in proportion to the sight and then it was dead on. They require a little getting used to. I went with an EO-Tech only because they make Mil-Spec sights. The glass is 3/16ths of an inch thick. The Bushnell Holo-Sight is similar, but not Mil-Spec. It is also less costly. I should add she is deadly with the thing. Pop cans are history out past 100 yards. Bill T.