Are you gonna drink that?
Many of the guns contain little to no metal, so they may be able to get through metal detectors without being discovered.
Metal detectors detect metal. A little metal, a lot of metal ... Doesn't matter. It's still metal. No gun will work without metal containing the controlled explosion of gunpowder igniting in a confined space. Thats how the bullet actually fires from the end of the gun (end of the muzzle is the path of least resistance) A lot of people were making the same types of ignorant comments when glocks came out.
As to these types of "guns" ... You'd never catch me trusting one to not blow up in my hands. What type of person would want a gun like this? It's not durable. It's not really safe. Those printers are expensive as hell. So, who would want one? Hmm ... Perhaps criminals wanting a disposable, non-traceable gun. For this reason, I'd be in favor of outlawing 3-D printed weapons technology. It simply serves no legitimate purpose.