The MR556A1 offers the battlefield-proven reliability, durability and precision you would expect from a rifle that bears the name Heckler & Koch. Like the HK416, the MR556A1 uses a proprietary HK short-stroke gas piston in place of the direct-gas-impingement system found on lesser AR15-style rifles that propels exhaust gas into the upper receiver to operate the action. In the "closed" HK piston system, exhaust gasses never enter the receiver, which means the HK runs cooler, cleaner and more reliably. When adverse conditions, like dust, sand and mud are introduced, the advantage of the HK system becomes even more drastic. The MR556A1 is assembled in the USA using US and German-made components and uses many of the same parts and assemblies developed for the HK416.