Inferior Rifles compare the HK MR556 A1 and A4 to show what is new with the A4. The MR556 A4, HK-USA's updated commercial version of the HK416, features a 5.56x45mm short-stroke gas piston system designed to reduce fouling and wear, an adjustable gas regulator for suppressed or unsuppressed use, and ambidextrous controls, while also incorporating a lighter, slimmer profile.