To drive the point home a little further. No one, at least in this context, is making an all plastic gun with a 3D printer. It simply doesn't happen. Even the memeworthy and incredibly janky Harlot 22LR uses steel barrel liners. It is also difficult (read: impossible) to have strong enough springs to fire a primer without using steel. Plus cartridges and bullets themselves are famously made out of... metals.
The notion of a 3D printed plastic gun sailing through a metal detector are pure fantasy. Completely fictional. Bogus, bunk, absolute bullshit.
But legislators believe it, because politicians are not actually experts in anything except playing politics. Which in general does not equip you with knowledge or experience from the real world.