Weaver rail and the requirement that it does not do any damage to the rail. That's why the Spectre has A.R.M.S mount on it too, that was a SOCOM requirement. SOCOM tested various optics mounts and found over time of taking optics on and off caused damaged to the rail and eventually made it out of spec. The worst they tested was Laure, it caused the most damage to the upper receiver over time.