Comment: na, as you know, all the metal parts must be purchased

separately.

And anyone can do that now.

as you know, metal parts like barrel, barrel nut, the bolt, the bolt carrier, pins for stress critical parts/springs, and other synthetic parts like o-ring, etc. obviously cannot be printed.

As far as speaking strictly of AR, I imagine once one starts playing around with AR uppers in higher calibers than 5.56, structurally, it may become an issue, but even with the stock mil-spec direct impingement setup, the lower receiver is not really going to be an issue, if the current 3D printable resin is somewhat nominally heat resistant. Though perhaps for sustained full-auto/select fire that may become an issue, but for semi-autos it probably won't be.

Regardless, for instance, I wouldn't dare put a .50BMG bolt-action upper mated to a printed AR15 lower. LOL!

The rear 'hook,' the buffertube thread portion of the AR lower receiver will feel the brunt of the stress; at least with current printable resins, not sure if they can can handle recoil from any round above anything between 5.56 ~ 6.8mm. Though would be curious to see if it will survive an extended session with a .50Beowulf upper!

UPDATE: HaveBlue noticed the same; here are some computer stress study pix:
http://haveblue.org/wordpress/wp-content/uploads/2012/08/15l...
http://haveblue.org/wordpress/wp-content/uploads/2012/08/15l...
http://haveblue.org/wordpress/wp-content/uploads/2012/08/buf...

From my limited past experience with some polymer injection molding, a right combo of elastomer added in like nylon, it may survive. But who knows, until it can be tested in full.

Regardless, I have no doubt, with time, the material stress issue would be resolved.

I read recently, can't remember whom, that they've already begun experimenting around with a .308 AR10 lower, could be HaveBlue, the same fella with the 3D-printed .22LR AR lower.

Calvary Arms and New Frontier Arms have already proven that you can make competent polymer AR lowers with no issues. So I don't see why the currently available tech and printable resin won't be able to handle at least the recoil stress from a 5.56 NATO round at bare minimum.

With a piston AR upper, I would postulate that there should be no issue at all, even with the current printed AR lower designs, as piston uppers would not have to contend with hot gas being blown back into the lower receiver via bolt carrier's gas key.