Part of me is somewhat worried. Part of me recognizes that a 3D printed firearm is hardly very practical, consider it's easy enough to get a real firearm, even for convicted criminals. (I'm speaking of the US here.)

What keeps people in the US from going on shooting rampages is not the difficulty of getting guns. It's the fact that most people don't want to go on shooting rampages. And while I'd much prefer it if guns were banned in this country, in the end, I'm not planning to lose any sleep over it. I'm far more likely to die in a car accident, even without adjusting for risk factors like where I live or my lack of involvement in criminal activity. So if I want to worry about something, I'll focus on driving more carefully.

Anyway, there's precious little you can do to stop 3D printed firearms from eventually being available. Betting against progress is rarely safe, especially in this domain where it's pretty obvious how things are going to go.