Talked with MGI today. The Takedown upper does not come with any barrel, kinda as I figured. The idea is that you can attach any mil-spec barrel, and
toss in a matching bolt carrier and mag, and have whatever caliber that you want.
I'm gonna get one of these from the ARFCOM store and try to do the briefcase AR thing. I don't know what calibers yet, but I'll post picks of the project.