HK 81000828 MR556 A4 5.56mm 16.5" 1/2"x28 Bbl Semi-Auto Rifle w/(1) 10rd Mag The HK MR556 A4 Rifle 81000828 is a 5.56mm semi-automatic rifle featuring a 16.5" barrel with a 1/2"x28 thread pattern. Designed for high performance and reliability, it closely mirrors the military HK416 platform but is civilian-legal. It includes HK's proprietary short-stroke gas piston system, a heavy-profile, cold hammer-forged barrel, and a slimline M-LOK handguard. This HK 81000828 comes with one 10-round magazine. The HK MR556 A4 is a high-performance, exceptionally smooth-shooting, and ruggedly durable rifle designed for serious operators and enthusiasts alike. While it is a semi-automatic variant, the MR556 A4 is the closest civilian-legal counterpart to the military-grade HK416 available in the United States. The primary differences between the HK MR556 A4 Rifle and the HK416 lie in the barrel design. The MR556 A4 features a heavy-profile, unlined barrel, compared to the thinner, chrome-lined barrel of the HK416. Despite lacking a chrome lining, the MR556 A4’s barrel is remarkably durable, exceeding military specifications during testing and achieving a round count of 12,000 rounds without failure. This performance is due to its German-engineered, cold hammer-forged construction using cannon-grade steel. True to its HK416 heritage, the MR556 A4 comes equipped with slimline furniture, including a narrow-profile handguard with M-LOK attachment slots for modular accessory mounting. The rifle retains HK’s proprietary short-stroke gas piston system—an essential feature that replaces the traditional gas tube with a robust “pusher” rod. This piston-driven system significantly reduces fouling and heat transfer into the receiver, virtually eliminating malfunctions often associated with direct impingement designs. It’s a key reason why the HK416 platform has earned widespread adoption among elite military and law enforcement units worldwide. Additional enhancements on the MR556 A4 include an "over-the-beach" style buffer tube with drainage ports for improved reliability in adverse conditions, and a reinforced double-length castle nut to ensure long-term structural integrity. Features of the HK MR556 A4:Closest you can get to an actual HK 416 in United StatesHeavy profile barrelM-LOK attachment systemGas-piston systemAmbidextrous safety HK MR556 A4 5.56mm 16.5" 1/2"x28 Bbl Semi-Auto Rifle w/(1) 10rd Mag 81000828