Heckler & Koch USA announces the availability of MR556 A4 upper receiver kits for sale at HK dealers nationwide. The MR556 kits are available in 11” and 16” barrel lengths and carry the same lifetime barrel wear warranty as a complete MR A4 rifle.
These are complete uppers with all components, including the assembled upper receiver, bolt carrier group, gas piston assembly, barrel, and adjustable gas regulator. They also are compatible with most common AR 15 / Mil Spec lower receivers.
The MR 556 is HK’s commercial variant of the legendary HK416. Its short stroke gas piston replaces the direct gas impingement system of a traditional AR pattern rifle. It runs longer and more reliably, with less maintenance and much less wear on internal components when compared to a DI system.
“We are proud to offer the A4 Upper Receiver Kits. Shooters can maximize performance of their current lower without the need to purchase a whole additional rifle,” said Nathan Schueth, HK USA Director of Sales & Business Development. “For existing MR owners, the URKs can be used to upgrade their A1 with the adjustable gas block or have a second length upper to change kit quickly and easily.”
The A4 variant of the MR556 rifle was announced at the beginning of 2025 and includes numerous upgrades over the previous A1 rifle, including completely ambidextrous controls, an adjustable gas regulator and a lighter barrel profile and slimmer handguard, that together trim approximately one pound off the weight off the previous complete rifle.
