did you guys see the big bust here in sacramento of AR-15 made from 3d printers.......the entire gun is legal to buy on ebay or gun store, no check, there is only one one piece called I think the upper? that must be bored out and put in. It is metal and the printer will make it for you overnight. Yes the fire too, very well. This is a real problem I don't know how they are going to tackle it. You could do this with a lathe and just the upper........a chunk of aluminum. Instructions are on the net.
https://ghostgunner.net/
250.00 for the printer. They also have a the liberator which is all plastic polymer.