BCG= bolt carrier group, yes?
Here you go: Carrier with bolt installed, and carrier with bolt next to it.
I saw your pic on akfiles but thanks very much for that.
Yep you have BCG right. That's Sig's bolt, carrier, charging handle, piston and piston cylinder. With that rail and cylinder, might be front heavy but should be reliable (if it's anything like the 556R gen 2).
Looking forward to seeing some reviews once it's in the wild.