First the original idea was a single shot throw away gun based on the WWII Liberator. However you can also make an AR upper from plastic that will shoot hundreds of rounds. All the other parts are not regulated, so basically it’s an untraceable gun.
https://www.youtube.com/watch?v=NF9kmPQCH98
Finally, you don’t have to print in plastic. Here’s a video of a fully functional 3D metal handgun.
https://www.youtube.com/watch?v=_EXsAeJ7RsU
Its actually the AR15 lower that can be printed. I would be cheaper and more durable to get an %80 lower and do the final machining yourself.