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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 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.
