The MR556A1 Upper Receiver Kit includes a complete upper receiver/barrel assembly, bolt carrier assembly, buffer, and buffer spring allowing most high-quality 5.56 mm AR-style rifles to be retrofitted to HK’s pusher rod system. 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 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 MR556A1 Upper Receiver Kit is designed to be fully interchangeable with all high quality AR-style firearms, simply by attaching it to an AR lower receiver using the standard takedown pins.

Due to the shortened length of the HK buffer and HK (action) buffer spring, these parts do not work in fixed stock AR-type rifles. Installation of MIL-SPEC hammers in non-HK lower receivers is required for reliable function. Additionally, the MR556A1 Upper Receiver Kit does not work in lower receivers with fully automatic or selective fire mechanisms.

However, the HK416 Upper Receiver Kit (available to military and law enforcement customers) is designed to function with fully automatic and selective fire lower receivers.

The HK piston driven operating system virtually eliminates malfunctions that are common to direct impingement gas systems 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.

Designed to function with a wide variety of high quality 5.56 x 45 mm ammunition, the MR556A1 Upper Receiver Kit uses a C.I.P. dimensioned chamber (a chamber with dimensions specified by the Commission Internationale Permanente). The MR556A1 also functions reliably with most quality Caliber .223 Remington ammunition.

The MR556A1 Upper Receiver Kit is produced in the USA using American and German made components. The Upper Receiver Kit uses a German-made barrel produced by Heckler & Koch’s famous cold hammer forging technology. The highest quality steel is used in this unique manufacturing process producing a barrel that provides superior accuracy and long service life.

The unchromed 16.5-inch barrel has an internal profile of 6 lands and grooves with a 1 in 7 inch (178 mm) twist. It is also moderately “swagged” with a slightly smaller internal diameter at the muzzle end than the chamber end. This feature has a positive effect on bullet accuracy and velocity. A thick, heavy contour also contributes to its excellent accuracy.

The Upper Receiver Kit also includes the HK Free Floating Rail System (FFRS) handguard with its four MIL-STD-1913 “Picatinny” rails. This premium machined alloy handguard allows all current accessories, sights, lights, and aimers used on M4/M16-type arms to be fitted. The HK rail system can be installed and removed with simple tools and returns to zero when reinstalled.

The Upper Receiver Kit also has a Picatinny rail machined into the top of its upper receiver for mounting optics and mechanical sights.

Part No. 81000584, UPC 642230262607 – MR556, Upper Receiver kit – includes bolt, buffer, buffer spring, rifle length M-LOK handguard and 16.5″ medium contour barrel assembly – MSRP $2,069