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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.
Complete MR556 Upper Receiver/Barrel Assembly— HK 16.5 inch cold-hammer forged barrel, upper receiver with forward assist, extended convertible charging handle, MLOK 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).
More Information
Barrel Length
16.5
Caliber
.223 Rem
Condition
New in Box
Finish Per Color
Black
Manufacturer
Heckler and Koch
Manufacturer Part Number
81000584
Model
MR556A1 Upper Receiver Kit
Type
Uppers
UPC
642230262607
WARNING: This product can expose you to chemicals including arsenic, which is known to the State of California to cause cancer. For more information, go to WWW.P65WARNINGS.CA.GOV
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