Strike Industries' Strike Rail is a lightweight, but feature-packed rail that you can depend on when entering harm's way. Its .57 internal diameter is for a suppressor or Oppressor compatibility. The rail is M-Lok compatible, has a built-in anti-rotation feature in the rail, ambidextrous QD socket placement, and reinforced steel mounting nut and bolt. It is not compatible with most gas piston systems. This 15.5 model has a blue finish.