Gen III forearm comes with 4 rails, 1 8.5" rail on top and 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 12.5" and weights 16.5oz. including barrel nut. The clearance from the center line of the barrel to the underside of the forearm is .78". The Inner diameter of the barrel nut is 1.075".