At this point in technology its a lot easier to make those using subtractive (traditional) machining rather than additive machining. Ask any gunsmith. You'll get into all manner of weird definition games, like the legal one for print your own is making a lower receiver, so are you cool with buying a COTS barrel and everything else but the lower? Or you want to make that stuff too, well, OK, are you cool with COTS firing pin and other very minor componenets? Or are you cool with buying plain old screws from home depot or do you insist on making those homemade too?