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 HKs 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
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.
Specifications and Features:
HK 16.5 inch cold-hammer forged barrel
Upper receiver with forward assist
HK Free Floating Rail System (FFRS) picatinny handguard
Extended convertible charging handle
HK Free Floating Rail System handguard
HK MR556 upper receiver
Complete MR556 Bolt Group Assembly
Tungsten Filled HK Proprietary Buffer
HK Proprietary Buffer (Action) Spring
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 dime nsions 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 & Kochs 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.