Originally Posted by onewheat
...but you can still only make an empty plastic lower receiver that has to be built and has a very limited lifespan - buy the upper, assemble everything and then what? Where is your advantage to being able to do this? It would just be easier to go and buy a complete AR that will fire thousands and thousands of rounds reliably, then to go through all of that hassle to fire a couple hundred (maybe) before it breaks. If you are worried about trace-ability - then buy an aluminum 80% lower (which is not considered a gun or serialized either), finish the machining and build a regular AR that has the reliability and is not traceable. I don't see where the 3-D printing is a realistically viable and practical option just to get around Big Brother - even for someone with a ton of cash. It is more of a novelty and a challenge to do it than anything to practically worry about.
The lower is the only part with a serial number, AFAIK. anyone can buy the rest of the parts and assemble one if they have the lower but the lower is, or will probably be, regulated. If AR-type guns are outlawed and people are willing to go to great lengths to get one/many, this is one way to go about it.
Gangs and other criminals don't buy guns for a lifetime, they only need them once, really. If it lasts that long, they got their money's worth, IMO.