I've touched on the magic of angular measurements before. Typically, you're going to run into one of two flavors: minutes of angle or milliradians. The quick version of this is to understand that a radian is another way to measure rotation around a point. A milliradian, sometimes called a mil or MRAD, is 1/1000 of a radian.
Not a lot of people know the SpecterOS 4x. You've probably heard of its heavier and more expensive sibling, the SpecterDR, though. The SpecterOS is the same optic, but fixed at 4x. It is lighter, with similar illumination, but it is more akin to the ACOG than a 1-4x scope. I really like this optic, as it has outstanding glass quality and an attractive look. But I'm not crazy about the mount.