H&K’s 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. Their 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 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. The Upper Receiver Kit uses a German-made barrel produced by Heckler & Koch’s 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. 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 Upper Receiver Kit also has a Picatinny rail machined into the top of its upper receiver for mounting optics and mechanical sights
