I also don't think that a person should be able to create their own weapon on a 3D printer either.
3D printed gun are very specialized tool for removing fingers and hands. They have no other practical use aside from the aforementioned amputation purposes.
I've never owned one. (A 3D printed gun)
So I can't comment.
As someone with experience in both firearms and 3d printing... I can say with confidence that brittle plastic such as ABS or PLA (the most widely used plastics for printing) with their low melting points (around 200C) would be less than ideal for making a firearm.
You can 3d print with metal as well.
Not with any machine that anyone would associate with what is commonly known as a 3D printer.
What are commonly called "metal filaments" are actually plastic filaments with a minute portion of metal powder mixed in. It's supposed to make the plastic look like metal (it actually doesn't) but provides absolutely none of thr structural qualities of metal.
If you're going to manufacture guns from CAD files, ypu would use CNC, which is like 3d printing in usage, but exactly the opposite, it takes away materials instead of depositing material, so, with the right machine, you can build in any material that is softer than the steel cutting tools.
These machines are common, they've been around for decades, and people have them in home workshops.
On the other hand, you can make perfectly usable firearms with common shop tools or even hand tools. The instructions, plans, and templates are for sale on Amazon.
If you wanted to make firearms by hand, nothing would stop you. People have been doing that for centuries.