3d printed guns are a lot less scary than a number of things that exist already that could do a lot more damage. Given their construction, they're only accurate to a few feet, and given their one-time use abilities, you're not going to do much with them before you get tackled by several angry co-passengers.
Were I an evil terrorist, I'd be looking much more at other, completely legal things. Items such as two-part epoxies could be used to make a crude but effective knife, lithium batteries are trivially short-circuitable to cause a fire, etc. But, such things are much less glamorous and scary than a plastic derringer that's about as dangerous to the wielder as anyone else.
It seems the existential threat to an airplane remains the same: cockpit access. Note that in some areas, taxis have more security than an airplane, in the sense that there's no way to escalate from passenger to pilot (dividing wall).
Even today, I see pilots open the cockpit during flight to use the toilet. Usually, the FA blocks the passageway with a cart. It seems plausible to me that there's a short period of time that a well-trained ninja-like attacker could leave their first class seat, and obtain cockpit access.
But I guess it'd be too expensive to provide full pilot separation.
Millions of flights a year are conducted safely with this apparently lax security. Perhaps the pilots simply weight up the minuscule risk and rate urinating in their trousers as the greater inconvenience.
Back in the 90s I still remember the day when a kind flight attendant came to my seat and took me to the cockpit of the 767 we were flying to Florida in.
Back then aircraft were occasionally hijacked, they'd be forced to divert or land somewhere and some ridiculous demands made before, more often than not, everyone was allowed to go home unhurt.
There was a pilot who wrote an essay on the uselessness of the checks. Sure, check for firearms, but nail files and so forth can be found on the plane. One of his examples was to pull up a strip of emergency lighting and break it - now you have a pointy thing to use as a weapon.
I believe it's still easier to suborn one of the cleaning staff, delivery vehicle workers, service techs, ground crew, or other trusted person to carry your weapons into the "secure" zone.
Could be at any domestic airport since once you get your badguy stuff inside the wall, you can carry it anywhere inside the system before using it.
There are a lot of potential attack vectors but I do not feel safe to talk about them in detail.
Ammunition is pretty obvious on xrays. It would be just as easy to smuggle in a real metal single shot pistol (designed for the purpose) as it would be to smuggle in a plastic gun.
Any single shot pistol is going to be a useless weapon. Less dangerous than a knife in my opinion. Knives don't need to be reloaded and have unlimited ammunition.
Were I an evil terrorist, I'd be looking much more at other, completely legal things. Items such as two-part epoxies could be used to make a crude but effective knife, lithium batteries are trivially short-circuitable to cause a fire, etc. But, such things are much less glamorous and scary than a plastic derringer that's about as dangerous to the wielder as anyone else.