The Delta forearm comes with 4 rails, 5.5" rail on top and then 3, 3" rails at 3,6 and 9 O'clock positions. The forearm is constructed of wrapped carbon fiber, with an aluminum barrel nut and collar. Total length of the forearm is 9" and weights 13oz. including barrel nut. The clearance from the center line of the barrel to the underside of the forearm is 1.1". The receiver and the rail on forearm are not at the same elevation. The inner diameter of the barrel nut is 1.075" Custom barrel nuts with larger ID for larger diameter barrels can be made.