An MR556A1 Upper Receiver Kit includes a complete barrel assembly, HK Free Floating Rail System handguard, complete bolt carrier assembly, HK buffer, and HK buffer spring.
The same operating system is used in the combat-proven 5.56 mm HK416 as well as the G36, XM8, HK417, and a variety of other Heckler & Koch arms. The Upper Receiver Kit is designed to be fully interchangeable with all high quality AR-style firearms that use retractable buttstocks and MIL-SPEC buffer tubes. Installation is accomplished by inserting the HK buffer and buffer spring and attaching the HK upper to an AR lower receiver using the standard takedown pins.
The Upper Receiver Kit is the core of the HK 5.56 mm proprietary gas piston operating system. It employs a piston and a solid operating "pusher" rod in place of the gas tube normally found in AR15/M16/M4-style firearms. The HK piston driven operating system virtually eliminates malfunctions that are common to the direct gas impingement system since hot carbon fouling and waste gases do not enter the receiver area. This is the same key feature critical to the success of the HK416 rifles used by leading military and law enforcement customers.
Like the MR556A1 rifle, the Upper Receiver Kit is made in the USA using German and American made components. The MR556A1 Upper Receiver Kit is classified as a part and can be purchased without a Federal Firearms License ( FFL). Suggested Retail Price is $2,195.