In America, the only part that most people would want to print is the serialized part of the firearm. In a Glock it's the frame (which is polymer in OEM Glocks), and in an AR, it's the lower receiver.

You can buy a Glock slide assembly, barrel, frame rails, trigger, etc anonymously and without any paperwork.

The same is true of ARs, you can get all of the parts except the lower receiver anonymously and without paperwork.

Also, unless you live in a state where the local laws explicitly prohibit it (like WA), it's perfectly legal to make your own firearms. You can even serialize and register them with the ATF if you want (but you are not legally obligated to do so). You don't need an FFL/SOT to do it.

I wouldn't do it if you don't know your local laws though... If you break the law or violate ATF policy, they'll shoot your dog and send you to prison forever.