The well-known HK416 has a civilian version called the MR556 (MR556A1). A lot of the same components and assemblies of the HK416 are also used in the MR556. This is an AR-15 pistol with a 5.56 NATO caliber. It provides the durability, accuracy, and dependability that you would expect from an HK rifle based on its performance in the field. The HK gas piston and pusher-rod operating mechanism, like the one in the HK416 and M27, is used in the MR556A1 in place of the gas tube typically found in rifles of the AR15/M16/M4 design. Since waste gases and hot carbon fouling are not released into the upper receiver to force the action, this closed system solves frequent problems with direct-impingement gas systems. The barrels of the MR556A1 are cold-hammer forged from cannon-grade steel of the finest quality. They are unlined for increased precision and exceptional service longevity under semi-automatic fire. They have six lands and grooves, and a 1:7-inch twist. For even more increased accuracy and speed, the barrel also has a taper-bore construction with an internal diameter that is slightly smaller at the muzzle than at the chamber end.