Die Hard 2

Urban Myth.
In Die Hard 2: Die Harder, Willis is talking to the airport authorities about the fight in the baggage terminals.
He says "He pulled a Glock 7 on me! You know what that is? It's a plastic gun made in Germany. It won't show on your airport X-ray machines, and it costs more than you make in a month!"

Aside from the fact that theres no such thing as a Glock 7. Theres no such thing as an undetectable gun. Knives: YES, guns: no.

Glock as a general rule uses more polymers than any other commercially manufactured firearm. But even in the back pages and obscure named weapon companies, an X-ray machine can see a handgun.
The only way S.A.S. and Mossad operatives pulled off "non-X-ray" guns was by making the parts look like something else. Think the golden gun from Bond.

Bullet casings are made of metal, recoil springs are made of metal and even though polymers are see through theres still enough metal used in ALL commercial handguns that an X-ray machine would go off like a Christmas tree.

Even if a handgun that uses caseless ammo were to be invented, AND it was loaded with a ceramic head, AND the powder charge was buried enough to not be sniffed out by the bomb sniffers, AND a gunsmith came up with a way replace the metal frame and spring polymer guns seem to need, and ... etc; you think the government would let that knowledge go public and risk it falling into the hands of the "EY-rabs"?

H&K soccom came in a standard .45acp 12 round clip, S&W made a revolver that held 3 20-gauge shotgun shells. Uncle Sam buys blueprints and shreds them.