In Die Hard 2, John McClane refers to a (fictional) Glock 7, a porcelain gun that fires only once, but is undetectable by X-ray. The ammunition would still be metal, resulting in detection by airport security X-rays.
The problem isn’t solved there. Plastic knives probably make it through X-rays undetected. Furthermore, TSA dramatically features, as only security theater can, the X-ray screens that TSA officials use to scan passengers. Patience and a few cheap flights could help a misanthrope test whether his weapons of choice are visible on screen. A better setup would hide the screens behind curtains, like old photographers, or to fork out the cash for the laptop privacy filters advertised inside in-flight magazines.