HK MR556 Upper Receiver Kit w/Bolt, Buffer Spring, M-LOK, & Barrel Assembly 81000584

HK MR556A1 Upper Receiver Kit 223 Remington/556NATO 16.5 Cold Hammer Forged Barrel Features: 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. HK MR556A1 Upper Receiver Kit 223 Remington/556NATO 16.5 Cold Hammer Forged Barrel Specifications: HK MR556A1 Complete Upper Receiver 223 Remington/556NATO 16.5 Cold Hammer Forged Barrel M-Lok Free Float Rail Flash Hider Complete MR556 Bolt Carrier Group Proprietary Tungsten Buffer Proprietary Buffer Spring 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).