HK

HK's MR556A1 Upper Receiver Kit allows most high quality 5.56mm AR-style rifles to be retrofitted to HK's pusher rod system which virtually eliminates malfunctions common to direct impingement gas systems since hot carbon fouling and waste gases do not enter the receiver area. Note: this upper kit does not work in fixed stock AR-type rifles, or lower receivers with fully automatic or selective fire mechanisms. A Mil-Spec hammer is required for reliable function in non-HK lower receivers. The upper receiver has a Picatinny rail machined into the top for mounting optics and mechanical sights. The German-made barrel is produced by HK's famous cold hammer forging process with the highest quality steel for superior accuracy and long service life. The thick, heavy contour barrel is unchromed and has an internal profile of 6 lands and grooves with a 1:7" twist. It is moderately "swagged" with a slightly smaller internal diameter at the muzzle end than the chamber end which has a positive effect on bullet accuracy and velocity. This MR556 Upper kit includes: complete MR556 Upper Receiver/Barrel Assembly with 16.5" barrel, upper receiver with forward assist, extended convertible charging handle, free-floating rail system handguard; complete MR556 bolt group assembly; tungsten-filled HK proprietary buffer; HK proprietary buffer (action) spring.