The moral panic about 3d printed weapons is complete nonsense. It's been trivial to make firearms out of hardware store and few practical skills. Back in high school we even made little black powder cannons in the metal shop, raiding fireworks for their powder and fuse and using ball bearings as the shot.
You don't even need steel pipe to make a shotgun, they're low enough pressure that for some types of shot you can even use copper pipe.
[1] https://www.amazon.com/Field-Manual-Survival-Evasion-Recover...
https://gunfreezone.net/wp-content/uploads/2016/03/improvise...
What does harder to stop even mean?
It strikes me as likely that a 3D-printed gun will not be the most effective way to accomplish almost any kind of harm for almost any kind of perpetrator, no matter how situated. And that gives me a little comfort, even if I wish my countrymen were less fascinated with weapons.
But they also won't pass metal detectors.
Guns made from metal stock will probably always be more reliable and cheaper than plastic ones.
The issue is that a disposable plastic handgun is less detectable and will leave no useful evidence behind as it can simply be discarded at the crime scene.
(Of course, that's probably hogwash as a plastic feedstock capable of being a gun is probably going to be equally as traceable.)