Hope no more, because both things are already quite possible. Metal printing 3D printers are still prohibitively expensive — like, go to your bank and take out a mortgage levels of expensive — but they do indeed exist. And you can print a lower receiver for an Armalite quite easily which is the only serial numbered and tracked component in the US. Upper receivers are available via mail order because they are not the regulated part (except in certain states), so have at it.

You didn't hear that from me, though.

Same amount of lethality

people could start 3D printing AR-15 ghost guns

I've heard from people that they've been doing this already, quite successfully, for years.