Sixxgun wrote:Khaos the S16 was a 6.5 not 7.62 the 6.5 is a necked down 7.62 anyway
I stand corrected. I've finally taken the time to clear up the conflicting reports on the chambering. Yes, it shoots a Grendel, which fires a 6.5mm bullet from a 7.62mm casing necked down to the same length as a 5.56.
Here's a picture of the Grendel 6.5 round for anyone who cares.

What I'm saying is that the bullet (and thus the mag, and the receiver) is wider than a standard 5.56mm, yet the same length, so there won't be any mags out there with the same dimensions. I think you're correct in using a 5.56mm receiver, as it will look closer to the real thing than using the 7.62.
It took a lot of digging to clarify all the shit that was posted in various places, and I apologize for not doing the research the first time.