“I don’t see anything in a gun that someone could not make on a current metal printer, except maybe the spring.”
Springs are cheap though, and pretty impossible to regulate. The real problem is the barrel. You might be able to print a metal barrel with the correct geometry, but it won’t be strong enough to use in an actual weapon. It may last a while, but eventually, that part will fail.
This is why I think that trying to manufacture an ordinary firearm with current printing technology is the wrong track. Instead, they should focus on a design that can function reliably for a short time, say a few hundred rounds, while being cheap enough to throw away after that. Just use it until it starts wearing out, then print a new one.
Yes, I have written this on other threads, the beauty of 3d printing is that you get to customize your own design. You don't need to copy a commercially available product if you don't want to, and that includes weapons as well. If they can't make a gun barrel, I'm sure they can make other weapons that people haven't thought up yet.