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 an…

Just my $0.02, but I don't think polymer FDM technology will ever be suitable for gas system components. So if you go down that path, you're looking at sophisticated DIY metal parts that it will put the design out of reach of most people. ECM may be viable for some of these parts, but there's a pretty big skill gap between cutting a barrel tube to length (where actual length doesn't matter at all to the process), and cutting bar stock to the rou…

Ok, I posted like an id10t with info you already had. Here's take 2:

I have been referencing the Rebel9/Rogue9 packages, which have ECM v3.64. But you're right, the mandrel for .380, which should have a twist of 1:16, isn't provided. You could still use the 9mm mandrel with its twist rate 1:10. I'm not sure how much of an effect that increased twist will have on accuracy. It might destabilize the bullet more quickly over distance. That's someth…