The KDG HK 556 rails are a direct replacement of the factory HK 556 rail systems. Installation is easy, taking only minutes because it utilizes the factory barrel nut. These rails are lighter, longer, stronger and more ergonomic making them the superior option for your HK. The KDG HK556 rails come in two lengths a 10 inch and 13.75 inch available in black and brown type 3 hard coat anodizing. SPECS: Overall length: 10" or 13.75" Weight: 10.2 oz or 13.6 oz Finish: Black anodize type 3 hardcoat or brown anodize type 3 hardcoat Material: 6061 T6 aircraft grade aluminum Compatibility: All HK416 variants including 416, MR556, and M27. Mfg: Kinetic Development Group Llc