Yoiu have to open up the bolt face on a .223 bolt for use with 9mm or 7.62x25. I used a galil bolt on my straight pull build. Also if you decide to go with a blowback design do not lighten the bolt as you need all the weight you can get to keep the bolt forward during firing as the 7.62x25 was originally designed for open bolt machine guns. I filled the back of the carrier with lead on my MPK-25 build and used a standard AK47 recoil spring.
Originally Posted by vz58
The only comment I'd, hesitantly make, having not made the gun; is the desire for reversibility.
In my experience that's as easy with a custom gun as with a marriage. And it shouldn't be an initial objective or it certainly will be a final outcome.