The only thing you need to make is the "lower" or whichever part the ATF constitutes as "the firearm" I've seen someone take a shovel and turn it into an AK. Once you have the "firearm" part of whatever gun you're building, the rest of the parts can be shipped to you in most of the country (idk about CA, and NY though) and you can easily assemble the rest of the gun.

Like you say, you just need to build a key metal piece, and voila, the rest is buying parts that can be delivered to you, in some cases fully assembled.

You could also just buy black powder guns directly to your home (idk about in CA or NY though) which are not treated as "firearms" by the ATF.

The only people shooting 3D printed guns are enthusiasts usually, who have other guns.