A direct descendent of the HK416, the MR556A1 is a semi-automatic rifle developed by Heckler & Koch as a premium level commercial/civilian firearm with match rifle capability. Like the HK416, the MR556A1 is a major product improvement over conventional AR-type carbines and rifles.

The MR556A1 uses the HK proprietary gas piston operating system, employing a piston and a solid operating ‘pusher’ rod in place of the gas tube normally found in certain firearms. This method of operation virtually eliminates malfunctions that are common to direct impingement gas systems since hot carbon fouling and waste gases do not enter the receiver area. This is the same key feature critical to the success of the HK416 rifles used by leading military and law enforcement customers.

The MR556A1 also has a Picantinny rail machined into the top of its upper receiver for mounting optics and mechanical sights. Unlike earlier prototypes, major subassemblies, including the MR556A1 upper receiver/barrel assembly, are fully interchangeable with other high quality AR-style firearms. MR556A1 upper receiver kits can be used to retrofit competing legacy AR systems.

Like the famous HK416, the MR556A1 uses a German-made barrel produced by Heckler & Koch’s famous cold hammer forging process. The highest quality steel is used in this unique manufacturing process producing a barrel that provides superior accuracy and long service life. But unlike the HK416, the MR556A1 does not use a chrome-lined barrel. Chrome-lining can sometimes mask bore imperfections and negatively affect accuracy. For the new series of Heckler & Koch semi-automatic MR rifles, HK designers and engineers believe best accuracy comes with an unlined bore. The unchromed barrel has an internal profile of 6 lands and grooves with a 1 in 7 inch (178 mm) twist. It is also moderately ‘swagged’ with a slightly smaller internal diameter at the muzzle end than the chamber end. This feature has a positive effect on bullet accuracy and velocity. The thick, heavy contour 16.5-inch MR556A1 barrel also contributes to its excellent accuracy.

A precise enhanced two-stage trigger adds significantly to the accuracy of the MR556A1 and an extended latch charging handle makes operating the rifle with a telescopic sight easier. The retractable buttstock can be locked into a solid, non-moving position by tightening an Allen screw in the bottom of the stock. Special target rifle takedown pins ensure the upper and lower receivers are firmly mated for accurate performance.

Designed to function with a wide variety of high quality 5.56 x 45 mm ammunition, the MR556A1 uses a C.I.P. dimensioned chamber (a chamber with dimensions specified by the Commission Internationale Permanente).The MR556A1 also functions reliably with most quality Caliber .223 Remington ammunition.