More importantly, what is the barrel made out of? Yes, I know there’s some fully printed guns… but my understanding is that those are basically 1-time use and even then it’s questionable how reliable that single use actually is…

If you want something resembling an actual gun (more than one shot, won’t blow up in your hand, some reasonable chance of accuracy, etc), then you’re going to be using multiple metal components (including the bullets of course) all of which would show up on a metal detector.

And I'd argue that shell casings are probably harder to manufacture than a fully working firearm. The equipment needed to manufacture working ammunition end-to-end is pretty serious.

And importantly the barrel. Plastic cannot contain the pressure required to fire a bullet.

In the movies, you hide the bullets in a pen or something, and it bypasses the metal detector along with the keys, phones and watches.