For AR-15-pattern rifles, especially those without full top rails, one of the challenges in mounting optics is achieving proper eye relief. Add in the necessity of backup sights, and it can be difficult to fit everything you need onto a flattop receiver.
Leupold's IMS (Integral Mounting System) makes it easy. The mount places the riflescope high enough to clear backup sights and to co-witness even with standard A2 front sights. The cantilever design allows a wide range of eye relief, so if the scope mount needs to be placed further on the rail to allow room for irons sights you can still achieve the correct eye-to-objective distance.
The aluminum IMS cantilever design mounts directly to the your firearm's mil-spec rail to raise the scope and push it forward. This obviates the need for risers or specialized optics with shortened eye relief, and provides a solid, stable mount for a red-dot scope or magnified optic.