It is perfectly legal to make your own lower or firearm for personal use, and is done all the time.

A very simple mill that runs off a PC will let anyone with a bit of computer knowledge convert the public domain schematics into a file and into a program.  So, yes, teenagers are quite capable of doing so.

I think they’re still ironing out the kinks in it though, so who knows what the future will bring?

It’s well established that plans for weapons — be they guns or be they nuclear bombs — are legal. The issue comes when you build it or start selling the parts. So if Shapeways decides to print them it might be an issue, but distributing the CAD/CAM files is perfectly legal.

I’ve often wondered whether it would be legal to sell a complete gun kit made out of hard jeweller’s wax. Of course it would not be possible to shoot a bullet out of a wax gun, but you could convert that wax gun into metal (although home 3D printing negates that thought experiment I suppose)…

Yes.  Yes it is.  The error he made was so egregious, so obvious, and so dangerous that the folks who know anything about it will react instantly to correct the situation.

That particular error is roughly analogous to a life-or-death safety problem on the firing line.  You stop everything, including reading further down the page, and correct it RIGHT NOW.

Example?

At a particular state championship pistol match some 30 years ago, the dog belonging to the range owner got loose and freaked out for whatever reason.  He ran straight downrange, into the line of fire of twenty competitors.  I was just a kid compared to the average competitor but I didn’t hesitate.  In the middle of a firing string, with a season’s worth of competition at risk for every person on the line, I yelled “Cease fire!” as loud as I could.  The guy calling the match jumped three feet out of his chair but, without hesitation or any understanding of the problem, he instantly echoed the correct emergency commands of “Cease fire!  Cease fire and make the line safe.”

Then he looked at me for an explanation which I didn’t have to provide.  The owner started to yell cease fire, pointed at his dog running in front of all those pistols, and waited for the line officer to declare the line safe. Once that was done, he received clearance to retrieve the animal.  (Fun to watch, btw, as the field of fire was about 200×100 meters and the poor guy had to run all over it to get his dog back while everyone in the sport stood at the line and watched.  Even the people who had had their concentration destroyed by the unscheduled cease fire eventually started laughing.)

Over the rest of the day several of the oldtimers stopped by to thank me for not hesitating and doing the right thing.  None of them wanted to be the one to shoot that dog and it was only by dismissing all other concerns and immediately taking corrective action was that sad outcome avoided.

It’s the same way with the error about lowers vs. uppers.  That’s such a huge mistake that anyone who enjoys shooting and doesn’t want to see fellow hobbyists in jail will instantly dismiss with all other concerns and yell “Cease fire!” or, the equivalent in this case, post a correction.

“Further to other answers which say there’s no copyright/patent current on the AR15,”
["Further to" here means "what I am about to say builds upon and agrees with". I can see that it could be read as "in response to". I should have been clearer, though, that I was accepting that it was not licensed, and was taking the conversation on from that point, and exploring the licensing issues that are bound to crop up with newer weaponry (and other craftable things).]

“it’s only legal to make a gun for your own personal use if you’re not a licensed gun manufacturer.”
[Urgh. I'm sorry, this was unforgivably poorly phrased: you could read this as "it's only legal if you're unlicensed". Would have been better phrased as: "if you're not a licensed gun manufacturer, then it's only legal to make a gun for your own personal use."]

If you are a licensed gun manufacturer, then yep, you’ll probably need to also get licenses off the design companies.
[I was not responding here to the AR-15 issue, but to the possibility of 3D printing requiring license for newer patented designs, since to me this is the really interesting part of the discussion, and is going to become the stuff of nightmares soon enough if they start applying video and audio piracy law to craftables. I should have been clearer, once again.]

["If you're an unlicensed gun manufacturer/salesman then you're already breaking more serious laws, and likely won't care."]
This was the only sentence I wrote that I think was at least somewhat clear. Sorry again.

The AR-15 is kind of the Lego system of firearms, but just about every variation centers around basically the same lower receiver.  The same can not be held true of the upper.

After that, however, the missed points abound. Sure, most printers print in plastic, and most gun parts prefer to be made out of metal. But some printers do work in metal and some plastics might be up to the job. The real question is what, if anything, is different about a world where there are boxes wholly indifferent to whether a spatula or a pistol come out when you open them?

I tend to suspect not. There are whole neighborhoods in the Phillipines that produce perfectly servicable SMGs, assault rifles, and pistols with hand tools, so while it certainly is easier to print a gun than file a gun, the capacity to produce firearms in your garage is fundamentally not new. One can imagine that a Hollywood plot where the underground is full of untraceable guns sold in the gingy alley behind Ernie’s Replicators, but the fact is that said alley is already full of guns bought and paid for from factories of the usual type, and as much fun as I have with printers, (and as loathe as I am to needle my fellow makers) I tend to suspect that printers will fill more of the role of the often-dusty table saw than a replacement for the global manufactured goods supply chain. The status quo would seem to predominate.

ETA – I enjoyed your dancing last night.

As well, the frame, or serialized, regulated part of a glock, is made of polymer.  There are metal springs and such mounted on the polymer, but the frame itself is polymer.  the barrel, slide, and recoil spring are metal.

I own a glock 22, glock 27, previously owned a 17, 30, and 19.  I own multiple AR’s, rifles, pistols, and shotguns.  Feel free to ask for correct information.

Joshua