MR556 A4 Upper Receiver Kit - 16.5″ - 5.56 - M-LOK

H&K MR556 A4 16.5″ 5.56 M-LOK Upper Receiver Kit

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. 

Compatible With:

• HK MR556 A4
• HK MR556 A1***

***The MR556 A4 upper receiver kit is backwards compatible with A1 MR556's, however, the A4's updated dual/ambidextrous magazine and bolt catch releases are only functional with the A4 receiver.  The standard single-sided magazine and bolt catch release functions are fully compatible with the A4 URK mounted on an A1 receiver.