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.
[1] It's single shot, uses a low-powered cartridge, and is a bitch to reload; calling it a handgun is an extreme stretch.
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.
Shotgun shells are mostly plastic.
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.