This kit has been designed and manufactured to HK's exacting standards of excellence and is intended for use in, and with, Mil-Std buffer tubes, as are provided with the HK MR556A1 rifle. Due to the precise nature of the components of this kit, they must be installed as a complete kit to ensure proper operation. The MR556A1 Upper Receiver Kit (URK) consists of an MR556A1 upper receiver/barrel assembly, complete bolt mechanism, and buffer and action (recoil) spring. The MR556A1 Upper Receiver Kit is designed to be fully interchangeable with most high quality AR-style firearms. MR556A1 upper receiver kits can be used to retrofit many non-HK AR systems by replacing the aforementioned components with MR556A1 URK components.