Comment Well... (Score 4, Informative) 846
A synthetic AR-15 lower receiver is nothing new, printing one yourself is however. Is it dangerous? Yeah kinda, unless you a printing with a rather high strength polymer. Is it illegal? Not if you follow the BATF guidelines. Hobby machinists have been milling them from aluminum for ages. You can buy all sorts of jigs and receiver blanks. If you were to say start manufacturing lower receivers, or do not qualify to legally own them and get caught the penalties are very severe. Also as anyone who builds their own AR-15s will tell you, certain parts are in very high demand, you may wait weeks to months for something simple to complete your build. So I wouldn't worry about somebody building a ton of "off books" rifles any time soon.