The 4-16x is the adjustable range of magnification power (fixed power optics will just have one number like 16x50) and the 50 is the diameter of the objective lens (ft lens - in mm). When it comes to AR's I like to keep it as light and compact as possible and theyre better suited for close to midrange shooting so the leupy (or something similar) has my vote there. This may be helpful in making your selection: http://www.snipershide.com/forum/ubb...91#Post1938091