HK MR556 A4 Upper Receiver Kit (16.5")

The MR556 A4 Upper Receiver Kit is the complete HK416-pattern top end sold as a standalone assembly, and HK sells it as compatible with most common mil-spec AR-15 lowers, including prior-generation MR556 lowers. Verify receiver geometry and hammer clearance on a non-standard lower before buying. The 16.5-inch cold hammer-forged barrel carries HK's 1:7 twist with six lands and grooves, and it runs a short-stroke piston with a pusher rod rather than direct impingement, so combustion gas never enters the receiver. The adjustable gas block lets the action be tuned for a suppressor without swapping parts.

The kit is genuinely complete rather than a bare receiver: HK includes the assembled upper receiver and barrel with a forward assist, the bolt carrier group, the gas piston assembly, the extended convertible charging handle, the adjustable gas regulator, the tungsten-filled HK proprietary buffer and action spring, and a soft carry case. The HK free-floating rail is M-LOK with a full-length Picatinny top rail and QD sling cups. At 4.20 lbs the assembly is heavier than a comparable direct-impingement upper, which is the cost of the piston system.

This is the cheaper route into MR556 hardware if you already own a quality AR-15 lower, and it matters more now than it did: HK USA temporarily closed its 2026 MR556 order books on July 2, 2026, so complete rifles are allocation-limited.