Fearmongering. While the gun may be undetectable in certain situations (But seriously, not really), Bullets with their blatantly brass or steel cases, copper plated steel or lead projectiles, and incredibly explosive powder will NEVER make it past the same checkpoints.
And anyone without a Felony criminal record can legally manufacture guns in the United States, whether it's made out of plastic, steel, or space rock. All you have to do is fill out a short amount of paperwork and pay a small licensing fee.