I think once 3D printing gets good, there will be a lot of things more "legally interesting" than guns to cause regulatory issues.
Example: http://aresarmor.com/
Go head and read some of the info on this company, which is selling parts that are unfinished components of an AR-15 rifle. Essentially, they sell you an almost complete lower receiver (the part the ATF considers to be the "gun"), and you use some supplied jigs and a drill press to finish the job. Presto: untraceable, unserialized rifle unencumbered by federal rules on firearms regulated by interstate commerce.
Remember prohibition? They used to sell "grape juice concentrate" that could be mixed with water, stored in a jug for an appropriate period of time, and presto: wine.
All you need is a reasonably substantial proportion of the population willing to conspire with each other to avoid/subvert/ignore the law and you've got status quo. IMHO, restricting access to firearms will have a measurable, but ultimately negligible effect on death by firearms in the US.
It's not really an issue as long as some firearms (and thus some ammunition) is relatively unregulated; you could easily have a world where only single-shot hunting weapons are available, then use the ammunition from those (either intact or by disassembling and reloading) for other weapons.
Still, I aspire to owning an ammunition company; I wish ATK were for sale.
Basically the same thing was done in WW2.
Until 3D printers capable of printing firearms-grade metal, they aren't actually making it easier to for the vast majority of people to acquire firearms who would otherwise have to go through the regulated process. Of course, the entirety of the hysteria is based on the flawed logic that if everyone suddenly has easier(subjective) access to firearms, then everyone will start killing each other.
The reason people are looking at ways to "print" banned firearms parts is because they aren't available or are restricted in some fashion.