It is currently in use by some United States Special Operations Units as a replacement for the M4, and the Norwegian Armed Forces as a replacement for the AG3. It is also used by some European special operations forces, like the Polish GROM and the German KSK.
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.
The MR556 is the HK416, but with a semi-auto FCG instead of a select-fire FCG. There are other minor differences as well.
M27 Infantry Automatic Rifle (IAR)
Modified HK416 rifle developed for the United States Marine Corps Infantry Automatic Rifle program. This design won out, becoming the M27 IAR.