The MR556 upper receiver kit has been updated to include all the new A4 features, including new barrel profile, adjustable gas block, handguard and more. This kit includes a complete upper receiver/barrel assembly, allowing Mil-Spec 5.56mm AR-style rifles to be retrofitted to HK's piston and pusher rod system. This system utilizes a piston and a pusher rod in place of the gas tube normally found in AR15/M16/M4-style rifles. This closed system virtually eliminates the malfunctions that are common in direct-impingement gas systems, since hot carbon fouling and waste gasses are not propelled into the receiver area to operate the action. This is the same key feature critical to the success of the HK416 used by elite military and law enforcement customers. The MR556 A4 is assembled in the USA using US and German-made components and uses many of the same parts and assemblies originally developed for the HK416. It functions reliably and shoots accurately with a wide variety of quality 5.56mm or .223 ammunition. Due to the shortened length of the HK buffer and action spring, these parts do not work in fixed-stock AR/M16- type rifles. Installation of Mil-Spec hammers on non-HK lower receivers is required for reliable function.