HK MR556 A4 Upper Receiver Kit (11")

The MR556 A4 Upper Receiver Kit (HK part 51000385) is the 11-inch version of HK's standalone MR556 A4 top end, built to retrofit a mil-spec AR-15 lower with HK's short-stroke piston and pusher rod in place of the direct-impingement gas tube. Combustion gas and carbon stay out of the receiver, which is the property that matters on a short barrel behind a suppressor. The 11-inch cold hammer-forged barrel has a 1:7 twist and concentric 1/2x28 threads with a flash hider, and the adjustable gas regulator sets the action for suppressed or unsuppressed shooting without swapping parts.

The kit is complete: the upper receiver and barrel assembly with a forward assist, the MR556 bolt group, the extended convertible charging handle, the HK free-floating M-LOK rail with integral ambidextrous QD cups and a continuous Picatinny top rail from receiver to handguard, the tungsten-filled HK buffer and action spring, and an HK soft case. HK assembles it in the USA from US and German-made components.

Two fitment rules come from HK. The shortened HK buffer and action spring do not work in fixed-stock AR rifles, and a non-HK lower needs a mil-spec hammer for reliable function. On a lower with a stock, the 11-inch barrel makes a short-barreled rifle; on a pistol lower with no stock, it makes an AR pistol.