Heckler & Koch MR556 A4 5.56 NATO 11" Upper Receiver Kit

The HK MR556 A4 upper receiver kit converts a Mil-Spec 5.56 NATO AR-15 lower to HK's piston and pusher rod operating system. A piston drives a pusher rod in place of the usual gas tube, so hot gas and carbon fouling stay out of the receiver. It is the same system behind the HK416, and the A4 update adds a new barrel profile, an adjustable gas block and a new handguard.

The kit includes the complete upper with an 11" barrel threaded 1/2-28, forward assist, extended charging handle and free floating slimline handguard, plus the complete MR556 bolt group and HK's own buffer and action spring. The shorter HK buffer and spring do not work in fixed stock AR builds, and a non-HK lower needs a mil-spec hammer for reliable function. It comes in an HK soft case.