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.
HK MR556 Upper kit includes:
- Complete MR556 Upper Receiver/Barrel Assembly HK 16.5 inch cold-hammer forged barrel, upper receiver with forward assist, extended convertible charging handle, HK Free Floating Rail System handguard
- Complete MR556 Bolt Group Assembly
- Tungsten Filled HK Proprietary Buffer
- HK Proprietary Buffer (Action) Spring
NOTE: The MR556A1 Upper Receiver Kit is not classified as a firearm and can be purchased without a Federal Firearms License ( FFL).