HK MR556A4 Upper Receiver Kit 5.56x45mm NATO 16.5" M-LOK - AR-15/M16/M4

The HK MR556A4 Upper Receiver Kit is a factory complete 5.56x45mm NATO upper assembly from Heckler & Koch that drops onto any Mil-Spec AR-15, M16, or M4-pattern lower, converting a standard direct-impingement AR into HK's gas piston and pusher rod operating system.

At the heart of this kit is a 16.5" barrel with a 1/2x28 threaded muzzle, married to an M-LOK handguard that provides a slim, suppressor-friendly accessory mounting platform. HK's matte black anodized finish covers the upper receiver and handguard for hard-use durability.

The MR556A4 ships as a complete drop-on kit including the upper receiver assembly, bolt, buffer, and buffer spring - everything needed above the lower to run on a standard Mil-Spec lower receiver. An adjustable gas system lets the shooter tune cycling for suppressed use or varied ammunition loads, while the gas piston and pusher rod design offers an alternative to traditional direct-impingement AR operation.

This is a new factory HK component - the ideal upgrade for owners of a quality AR-15 lower who want to bring HK's piston reliability to their build.