3D-printed guns, good for just one shot, might really be dangerous.

It would be silly to make a fuss about the possibility of bringing such guns into airplanes or trains. There are other deadly weapons that current airport security won't stop, such as a strong thread with which someone might be strangled. With either weapon, a killer might manage to kill someone before being overcome by other passengers, but this danger is too small to be worth much protective effort. It's only barely worth worrying about bringing real guns onto an airplane, since the shooter couldn't crash the plane; if he just wants to kill some people, a shopping mall or city street is a softer target. However, assassinating an official with a printed gun might be a significant danger, and might call for new security measures on government offices.