Heckler & Koch announced this week that it will start offering MR556 A4 upper receiver kits at its American-based dealers this year. Options will include an 11-inch or 16-inch barrel, complete with HK’s lifetime barrel-wear warranty shared on factory-completed MR rifles.
Effectively, the move allows owners of most common AR-15 lowers to trim the fat on their HK budget by letting them make a centaur-like HK firearm that includes all the HK upper components: assembled upper receiver, bolt carrier group, gas-piston assembly, barrel, and adjustable gas regulator.
The kits are compatible with most common AR-15 lowers. Thus, buying one is a more affordable entry point for HK fans dreaming of their own MR556 A4 while living on an abbreviated budget. It also opens doors for owners of older MR556 models looking for an easy-button upgrade.
“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 and 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 MR556 is based on the renowned HK416, which is sadly only available to most American shooters as a .22 LR clone. The commercially available MR556 variant, chambered for 5.56 NATO, uses a short-stroke gas piston, making these upper kits a fast way to drop your direct-impingement system for a piston upgrade.
Along with offering a potentially longer life for your gun, HK’s piston system requires much less maintenance than direct-impingement systems.
The A4 model itself is new for 2025, so these uppers are on the front edge of HK’s current product offerings. Features include an adjustable gas regulator and a lighter barrel profile with a slimmer handguard. On a complete MR556 A4, that translates to nearly 1 pound of saved weight over the previous iteration.
HK’s new kits will include the complete upper receiver/barrel assembly, cold-hammer-forged barrel, forward assist, extended convertible charging handle, HK’s free-floating rail system, MR556 bolt group assembly, tungsten-filled proprietary buffer, buffer spring, and adjustable gas block.
They’ll come in an HK soft carry case and can ship to your door without an FFL required. The price tag is, well, still HK steep at an MSRP of $1,999 for the 11-inch upper and $2,499 for the 16.5-inch option.
