HD-5 551 a variant of the Eotech series sights. Built to the standard length and accommodates 2 x AG13 batteries. It has 12 brightness modes and can switch between red and green. It simply screws on and tightens to the 20mm rail.
This pro sight also has a protective rubber cover and optional fast release rail system. All in all an outstanding Eotech sight for Airsoft that will NOT disappoint!