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The HK416 is an assault rifle that bears many external resemblances to the M4A1 Carbine but is internally different. It features an HK proprietary short stroke gas piston system derived from the HK G36 family of assault rifles
The HK416 is very similar to the M4 in appearance, but is different internally. The primary difference between this weapon and the M4 series is the short-stroke offset piston system, replacing the expanding gas inline piston system usually found in AR-pattern rifles. The mechanism of the rifle is actually closer to the H&K G36, rather than AR-pattern rifles. The HK416 has a slightly larger diameter bolt (.53 overall diameter (OD), as opposed to .512 OD) than the AR-15 does; an AR-15 bolt will fit in the HK416 carrier, rather loosely.
The HK416 has a quad rail to mount a variety of accessories and can mount the AG36 grenade launcher in seconds without any modifications. It also comes in a 7.62x51mm version called the HK417 which makes it a rival to the FN SCAR-L and FN SCAR-H that are currently in trials with US SOCOM. An HK416 upper receiver can fit on any AR-15 lower receiver without modification.
MR556: The MR556 is the HK416, but with a semi-auto FCG.