Heckler & Koch's MR556A1 is the semi-automatic civilian version of the HK416, the gas piston rifle that replaced the M4A1 in several NATO military forces. It uses HK's proprietary short-stroke gas piston system instead of the direct-impingement design found in standard AR-15s. The 16.5-inch cold hammer-forged barrel has 6 grooves with a 1:7 twist, and the receiver is machined from aluminum with a Picatinny/M-LOK rail system.

At 137.6 oz (8.6 lbs), the MR556A1 is noticeably heavier than most 5.56 rifles in this category. That weight comes from the piston components, the heavy-profile barrel, and HK's overbuilt receiver set. The two-stage trigger pulls at 7.6 lbs, which is heavier than many shooters prefer. Owners frequently replace it with an aftermarket unit, since the MR556A1 accepts many (but not all) standard AR triggers. The rifle ships with a Magpul grip and an adjustable stock, but no iron sights.

The MR556A1 carries the direct lineage of the HK416, which gained fame after DEVGRU's selection and subsequent high-profile operations. Heckler & Koch built the civilian variant on the same production line in Columbus, Georgia, with the same barrel, bolt, and piston assembly. For buyers who want that military pedigree in a semi-auto package, nothing else on the market shares the same parts heritage.