Anyone start to make these yet? Anyone have an idea of what mag could be used in these?

There is only 1 mag that can be used. That is the FN 5-7 pistol mag.

I think a 7.62x25 upper would kick ass.

Enigma102083,
    I got a 7.62x25 barrel done by Marty at Teppo Jutsu and it is now part of the best pistol caliber upper I have. It runs smooth and never has any type of trouble. It uses the standard 9mm bolt and I found that a "H" buffer worked great with this ammo. There is even HP ammo available now for this round.

    I think that the 5.7x28 round would be good but the ammo (except for some new Winchester stuff) is not cheap! I think I would prefer a 22 Magnum or .17 HMR upper since the ammo can be bought almost any place but again magazines are a problem. Unless you can get a Grendel 30 round 22 Magnum magazine to work you are stuck with existing 7 to 10 round mags. I guess the problem is that not enough people want this stuff so no manufacturer wants to spend the time and money to develope it in the AR platform.

AR fan, what mags do you use with the 7.62X25?

Thanks
Paul.

Paul,
    I use PPS-43 mags and I leave the ridge on the back in place. I built a magwell adapter very similar to the Colt type magwell adapter except it has a curve on it to match the PPS-43 magazine. Due to PPS-43 mags being longer front to back, I decided to modify the rear piece that holds the ejector instead of the front ramp piece -- I wanted to keep a nice smooth ramp to the chamber.