Since you're pretty ignorant, let's go over the major parts of an AR-15.
Barrel.
Upper receiver.
Lower reciever.
The stock.
You can buy any of the parts without needing a background check of any kind except for the lower receiver. The ATF only tracks serial numbers and, guess what, the upper receivers don't have them! This means that you can purchase one without any form of background check if you so choose.
http://www.primaryarms.com/ar-15-uppers/c/101200/
So, we've established that you can simply buy an upper receiver. We've established, through previous posts, that you can simply make your own barrel (or buy one, if you wanted to!) A stock can be bought anywhere. So that leaves just the lower receiver. Shockingly, that can be 3D printed!!!!
https://www.youtube.com/watch?v=TlRN1C8t6n0&t=0s
https://www.youtube.com/watch?v=ZqvFW71g8gM
So in short, getting an unregistered, unlicensed AR-15 really isn't that hard. I don't own a 3D printer nor am I an expert on them so I can't speak on the longevity or toughness, for lack of a better term, on these 3D printed lower receivers. Magazines and ammunition are just as easy as the rest of the gun to come by. As you said, I "came into this conversation late" but I highly doubt anyone was implying that you can 3D print an entire gun. If they were, they're wrong. But then so are you.
Oh, and while we're at it, this is perfectly legal. You're allowed to make your own firearms.
https://www.atf.gov/firearms/qa/what...r-own-firearms
Hope that was on-topic enough for you and I wasn't "derailing xD" the thread.