A couple of things regarding this:

Federally there is no law that says you can’t print and carry your own gun, no serial number required.

However several states have strict laws and there are weird caveats. It has to be for personal use, cannot be sold or transferred and under the Undetectable Firearms Act any firearm that cannot be detected by a metal detector is illegal to manufacture, so legal designs for firearms such as 3d printed guns require a metal plate to be inserted into the printed body. Also online posting of plans for 3D-printed firearms require a license under the Export Administration Regulations issued by the Bureau of Industry and Security.

So if you design your own gun, or get one from a company that has an export license, print it and then ensure that it has enough metal in it to be detectable… Go for it, should be legal.