Fully DIY 9mm handguns aren't viable yet. Simple blowback designs like the FGC9 don't scale down because you need about 500g of bolt weight. Until someone comes up with a viable locking mechanism, PDW size is about as compact as you can get.

More modern DIY alternatives to the FGC9 include Rogue/Rebel and Urutau. The Rogue/Rebel designs are very full-featured, but making the bolt is more work than other designs. Urutau is a pretty easy build and a really enjoyable gun to shoot.

We'll hopefully be able to add the Decker 9 to that list soon.

Can you not just buy a barrel? If you live somewhere that's firearm adverse then isn't ammo gonna be your real problem? If you can get ram sets without difficulty then you could always make 26.5 mm from 1inch electrical conduit piping. Or 15mm or 37mm which in my opinion is just as fun if not more than regular firearms and aren't technically firearms in the US.