I'm in the process of evaluating the Faxon Firearms ARAK-21 upper receiver. It fits any mil spec AR lower, is a long stroke gas piston with a forward, non-reciprocating charging handle. I had a couple of malfunctions early on, of which a few appear in this video, but the problem has been resolved and it is functioning great. The full review and video will be out soon, just as soon as I get some .300 BLK ammo so I can test the 2nd barrel I got with it.