This is an interesting article - to summarize:
The Europeans are worried that 3D print guns can be made.
There are reported demonstrations that they would fire enough shots before breaking to be used in some onetime evil act.
They have been smuggled past metal detectors - see Israeli example.
Software solutions for printers might block such usage (although I would think that is an easy hack).
-- My thought - isn't getting the ammo hard in Europe. If you could get the ammo illegally - probably be able to get a gun.