The lightweight, streamlined design of the Daniel Defense A1.5 Fixed Rear Sight makes it the optimum back up iron sight for shooters using todays most popular reflex optics. Its well thought out design does not impede the use of the charging handle on M4 type rifles. The one piece design makes it easy to install to any Military Specification flattop upper receiver and the sight is secured to the upper using a slotted fastener which is threaded into a self-locking steel thread insert via use of Patented Rock and Lock Attachment Technology. Sight is set at Absolute Cowitness Height. Rear Sight Aperture Small: 0.070″. Rear Sight Aperture Large: 0.200″.