Heckler & Koch MR556 A4 Upper | 5.56 NATO | 11.5in | 51000385

The Heckler & Koch MR556 A4 Upper Receiver Kit is a complete AR-style upper assembly that adapts Mil-Spec 5.56mm lowers to HK’s piston and pusher rod operating system. It features an 11.5 inch stainless steel barrel and a free-floating M-LOK slimline handguard, and ships with a bolt group and HK proprietary buffer and action spring. This upper is suitable for range and training builds or duty-style AR-platform projects where a piston-driven system and modular handguard are desired.

Product Specifications

• The manufacturer is Heckler & Koch / H&K.
• The model is MR556 A4 Upper Receiver.
• The manufacturer part number is 51000385.
• The UPC is 642230270220.
• The caliber is .223 REM/5.56 NATO.
• The barrel measures 11.5 inches.
• The barrel material is STEEL.
• The barrel finish is STAINLESS.
• The finish is Anodized (Black).
• The handguard is a free-floating slimline M-LOK with integral ambi QD cups.
• Included features are forward assist, extended charging handle, and complete bolt group assembly.
• The kit includes a HK proprietary buffer and action spring.
• The muzzle is 1/2 x 28 suppressor-ready.
• The mounting interface includes a PICATINNY RAIL.
• The overall weight is 8.8 pounds.

This MR556 A4 upper kit delivers HK’s short-stroke piston architecture in a ready-to-install package for AR-15 style lower receivers. It pairs proven piston operation with a slim, modular handguard and is supplied with the bolt group and HK buffer/spring required for correct function with compatible lowers. The assembly is intended for builders and shooters seeking a durable, low-fouling operating system for range work, training, or duty-style rifle projects.