Heard through a rep that the 516 will be available in 300 BLK shortly after they finish their current orders for the M-400's after December of this year.
Been wanting a 516 so I will wait. Any of you 516 owners have any issues??

No issues with mine!

Glad to hear. I have several other AR's.  Have been interested in a piston gun simply because I do not have one. Have been dealing with the Sigs over the last year and the 516 has floated to the top due to its basic rail and the Sig provided sights. I figure a nice electronic sight and the 300 BLK cartridge will make it that much sweeter. The Rep visiting our range ensured me Sig is dedicated to this cartridge to the point they will add it to their cartridge line as well in 2015.