The part that's 3d printed is the lower receiver. In an AR-15 style design, the lower receiver doesn't handle any pressure at all, it just houses the magazine and hammer/trigger/sear/safety, and connects to the upper receiver/barrel assembly and the buttstock/buffer tube assembly.

People have made lower receivers out of aluminum basically forever. Aluminum could maybe be made to handle .22lr type pressures, but certainly not .223rem or bigger. A plastic lower is completely believable.

EDIT: to clarify, I meant if you wanted to make the whole gun out of aluminum you might be able to get away with it for a .22lr. Aluminum for a lower receiver is absolutely no problem at all. Apologies for not being clear.

The M16/AR15 upper and lowers in milspec form are made from 7075T6 (might be T651 or some other specialized heat treatment, I am not sure off hand). Others have made them from 6061 Al.

The chamber is part of the barrel, which is made of some form of steel. Thus, the upper receiever doesn't contain the chamber pressures. What it does do is control the bolt carrier assembly when it recoils, which is pretty darned vigorous - I wouldnt trust my face to a plastic upper.

Also, note that plastic firearms have been around for YEARS. My dad's first rifle, in the later 50s, was a Remington Nylon '66, which was except for a steel stamping for a cover, a steel bolt, and the steel barrel, made of Zytel.

Well there was this story (http://boingboing.net/2011/09/20/3d-printed-ar-15-parts-chal...) on BoingBoing in September of last year that was later determined to be a hoax according to the Makerbot folks at the Makerfaire this year.

If I recall correctly, the lower is also the only part of the assembled weapon that requires registration.

Incorrect. The lower is the only part of the assembled weapon upon which a serial # is applied and a tax is paid. It is also the # that is recorded on the Form 4473 at the dealer when you purchase the weapon.

There is no federal level firearms registration (excepting NFA items), and only a few states that do so as well.

Oh, and if you build it yourself (as this person has done), no tax need be paid, no serial # need be applied, and all perfectly legal provided you have not been restricted from purchasing a weapon through normal channels. That is, a felon making a weapon is now a felon-in-possession, where a citizen with full rights making one has not committed a crime.

One of the things we did in my machining class was we built miniature brass cannons. They were .177 caliber (In the US that is a common size of BB). They were fully functional. You put in some gun powder, a bit of wadding, and a BB, then you could fire them by putting a match to the touch hole.

Somewhat difficult to wield in a gun fight though, not that we didn't occasionally imagine the students holding off an attack by firing a broadside from cannon of various quality :-)

It is recommended that you do apply a serial number because LEO may incorrectly conclude that the firearm is illegal if they don't see one.

Yes, but there is no requirement to do so. Nor is that serial # available anywhere to look up.

  Aluminum could maybe be made to handle .22lr type 
  pressures, but certainly not .223rem or bigger.

?

The Alexander Arms .50 Beowolf lower is made out of aluminum. http://www.gunblast.com/50Beowulf.htm

You can also buy single-shot .50 BMG uppers that bolt right onto regular aluminum AR-15 lowers, though of course you can't feed .50 BMG through a 5.56 magazine well.

He did not "print a rifle", he printed what is known as a "stripped lower receiver," which is only a shell that holds the trigger assembly, magazine, grip, and provides a mount point for the buffer tube and two pin mounts for the upper receiver. The upper receiver and buffer tube/buffer spring are the objects which have the force primarily exerted on them.

In the eyes of the ATF he DID print a gun. The lower receiver is a 'gun' for the purposes of manufacturing. The upper receiver, buffer tube, trigger group, etc. are just parts...

I'm not sure what you're getting at. I think you're trying to be pedantic?

I said he didn't print a "rifle," you say "in the eyes of the ATF, he DID print a gun." I respond with, if you're being pedantic, he printed a "part which the BATFE classifies as a firearm requiring the stamping of a serial number should the manufacturer transfer it to any other individual." A part which, we all know, in and of its self is NOT a rifle.

During my studies we ssome classes in rapid prototyping. One of our favorites (since prototype car parts or funtional gear boxes aren't simply cool enough) was a fully funtional, for 20 shots or so, (semi?)-automatic rifle. And that was 6-7 years ago. The only non plastic part was the barrel, after the 20 shots the resin and plastics melted so.