9mm blowback, using a rewelded Apex Galil receiver. I'm still reading up on the trust and Form 1 stuff.
edit: Jack Squat PPSh-41 magwell, PPSh-41 mags. The PPSh bits look more similar to the Magal ones than the Suomi bits. Several people have mentioned the PPsh mags handled 9mm with no trouble.
The most difficult/expensive part will be the folding stock. The Magal uses an uncommon variant of the Galil stock, and I might wind up having to fake that unless I can snag a deal on the right parts.
Form 1 and trust went out yesterday. Supposedly a 15-month processing delay now... and mine will be delayed even more, since I just discovered that when I made copies of everything I put in the envelope, I mixed up the original and copied documents. One of the original Form 1s and the citizenship certificate are on my desk, and the copies are in the mail to the ATF. <sigh> Figure another month or two after the examiner opens the packet and sees my mistake.