A barrel inside a curling iron, the spring inside a hair dryer, the slide inside a laptop, the frame does have small metal bits but, depending on their scanning

A barrel inside a curling iron, the spring inside a hair dryer, the slide inside a laptop, the frame does have small metal bits but, depending on their scanning tech, probably wouldn't appear much like a gun. You probably don't even need bullets honestly, just the visual aid. You don't even need a gun though, a knife is plenty lethal on it's own and readily available inside the terminal and even on the plane.