Unless your building / buying a NFA Class 3 rig I’d ignore the hype and buy a good M4gery from Bushmaster and go with that. The Hk 416 is a great design for full auto yet after carrying and using assorted M16, M4, XM-177, GAU 5A/AA rigs for 26 plus years I can say the key is keeping the weapon clean and properly serviced. In civilian / retired status these days I keep a M4gery with ACOG and redimag in my vehicle and have taken many coyotes with it and used it at the range and 3 gun matches as well , rapid fire lots of rounds without cleaning..... never a failure to feed or fire thus far.
I’d buy the flattop bushmaster, add a redimag, a surefire forend light and a good assortment of 30 round magazines NOW as well as ammo !
A few key spares like pins and small springs that get lost more than they do broke during cleaning.
If you still want a piston rig this is one I have seen for civilian buy and personally fired trying to get it to fail .....good gas piston version.
As well a buy now and you can get a piston upper later if time allows and ya still want one !
Link to a good piston gun.
http://www.pof-usa.com/p415/p415.htm
....just my opinion and what I do and have experienced.
Stay safe and Good Luck !!
Piston big bump, the M16's extractor retaining pin broke on me in the field at dusk. It wouldn't extract. It wasn't a good night. I haven't liked gas impingement since. JMHO
Book mark for later edification.