The HK-USA MR556 A4 chambered in 5.56x45mm its short-stroke gas piston operating system does not impart hot carbon fouling back into the action allowing the weapon to run longer with less maintenance and with less wear on internal components than direct-impingement ARs. Its adjustable gas regulator allows the operator to tune the action specifically for suppressed or unsuppressed operation. Fully ambidextrous controls lighter weight and a slimmer profile make the A4 easier to operate manipulate and employ. Includes one 10rd magazine.