I should imagine the "scary" part for security services is the fact that it's printed in plastic and therefore much harder to detect, not that it's home-made.
Nothing stops you from milling and turning plastic. Not only that, there are many heat resistant plastics on the market that are far stronger than some poorly bonded ABS.
The Liberator derringer[1] which caused such a fuss a few months ago only needed a nail, which wouldn't be detected under most circumstances. I imagine that using an epoxy pin would probably be suitable given the gun's other single-fire characteristics.
[1] It's single shot, uses a low-powered cartridge, and is a bitch to reload; calling it a handgun is an extreme stretch.
They put in a metal firing pin because 100% plastic guns are illegal in the US. It was technically capable of being completely plastic.
But like the other poster said, it's a one-shot pistol with a weak cartridge. It's a long way from a practical firearm that will survive more than 10 shots.
You make a good point with flare guns, but I wouldn't be terribly afraid of them. Maybe someone on a plane could cause a lot of panic with flare gun but that's not much different that causing a fire with other means. You might be able to kill someone with the initial shot, but that's it.
In a shotgun shell, the pellets (or slug) and primer section are still metal. You couldn't smuggle a shotgun shell through a metal detector any more than you could smuggle a regular bullet through.
