An AR with a gas piston is sort of like a woman with a trouser snake. You can still call it an AR, but accepting it as one requires a bit of a stretch.
I mean, it's described as a development of the m4 carbine, and it shares a caliber, but the entire gas system is proprietary h&k... They also redesigned the grips, rail, and sight.
Not saying it's bad. H&k makes nice stuff. But it's a fundamentally different system. I'll give them compatibility, though, and this was a smart marketing move. Supposedly you can mate the entire 416 upper with an m4 lower.
I note that this has moved into us service and is reputed to be ultra-reliable. Aka "the gun that killed bin laden"...