Some random thoughts...
-3d printers can be purchased for $500 or less.
-building improvised firearms isn't new, and it isn't complicated. "zip guns" have been around for ages.
-this latest thing isn't even the first 3d printed firearm design to go around. 5 years ago a design called "Liberator" was circulated around the world on Bit Torrent. It didn't lead to a rash of shootings. The "Liberator", in practice, was found to suck pretty bad. Unreliable, inaccurate, and prone to break.
-gun-geeks, NRA types, mall-ninjas, and so on, have no interest in 3d printed guns. Picture how Harley riders and custom motorcycle enthusiasts feel about motor-scooters, and that's about the way these guys guys would feel about 3d printed guns.
-the NRA receives a lot of its cash from manufacturers, who would prefer that you buy their product than print your own.
-regular street criminals would be unlikely to use a 3d printed gun. First off, they'd need something reliable. Second, they're not exactly known for their 3d printing skills. Third, they seem to have no trouble getting real actual firearms anyway.
-even somebody who couldn't get their hands on a real gun would still be better off building a trusty "zip gun" than using a 3d printed gun.
-so who would actually use a 3d printed gun? I think two distinct groups of people. One, people who are really curious and interested in experimenting. 3d printing geeks, hobbiests, inventors, experimenters, this sort of thing. Somebody who has no interest in actually committing crimes, but is interested in the idea of trying to make this work. The second group is people who would want a gun that is invisible to security measures. Hijackers or somebody who wants to commit a murder inside a secured venue. This isn't going to be a "rampage" weapon... the shooter is likely to get one shot or less.
To summarize, I don't think this is actually anything to panic about. Designs for more conventional improvised firearms are already available on the internet. Designs for bombs are available on the internet. You can't put the genie back in the bottle. The availability of the information hasn't actually caused violence. Like sales of dealcoholized beer or NDP memberships, widespread availability hasn't resulted in widespread demand. People seeking a security-invisible gun may have a new option, but that's already an incredibly tiny market.
-k