The E is for Extreme. The EXPS3 offers true 2-eyes-open shooting and a 7 mm raised base that still allows for iron sight access. It features easy-to-adjust side buttons and an adjustable locking quick-detach lever for easy attachment and removal. EXPS3 was chosen by the U.S. Special Operations Command (USSOCOM) as their primary optic for close-quarter engagements. It is also the optic of choice for the technologically advanced modern rifle hunter. Its night vision-compatibility makes it a versatile choice for day or night hunting. Product Details: Night vision compatible – Length/Width/Height: 3.5 x 2 x 2.4 (91.4 x 50.8 x 60.9 mm) – Weight: 8.0 oz (227 g) – Waterproof: Submersible to 33 ft. (10 m) depth – Mount: 1 Weaver or MIL-STD-1913 rail – Return to Zero: Repeatable to within 1 MOA after re-mounting – Brightness: 20 daylight settings 10 additional settings for Gen I through III+ night vision devices – Power Source: One 123 lithium battery – Battery Life: 600 continuous hours at nominal setting 12