It wouldn't be hard at all to sneak a few bullets on a plane. Have you flown recently? At many airports they try to force everyone through the body scanners, but if you refuse, you bypass both the body scanner and the metal detector and get a non-invasive patdown that wouldn't be sufficient to find a couple .22 bullets. The harder part would be assembling and loading the thing once you're on the plane without someone noticing.
I don't think it's true that opting out always bypasses the metal detector. I preemptively opt out every time I fly, and I think only once did they take me around the metal detector. I remember it because I was so surprised.
Why would you be on the plane? This is getting it through security.
Get enough of them through security and one can eliminate security.
What happens next is no longer constrained by "what one can get through security."
But to answer your question more directly, an airplane is one of the few places where a zip gun could actually be effective as a firearm. The cockpit door is fortified. Is the galley bulkhead or the wall of the lavatory?
What about that period of time when one of the pilots has to use the lavatory and the 5'2" FA is guarding the front of the plane during egress and ingress? At that point you're relying on human tissue to prevent access to the cockpit.
Perhaps you're thinking of a zip gun as something that shoots rubber bands, but what you'd do is fashion a firing pin out of the smaller cigar holder and plunge it into the larger cigar holder to make the bullet come out the other end, or any variation on those mechanics.
What I'm saying is that this doesn't really matter. Cockpit doors lock now, and waving a gun isn't going to make the pilots open them.
If terrorists just want to shoot somebody, there's no reason for them to bother getting on an airplane.
The most such an attacker could hope to accomplish would be to take someone hostage and negotiate over intercom for the plane to land at a different airport, the way hijackings used to work. This is undesirable, but not a threat of nearly the same magnitude as the 9/11 attacks. It barely even makes the news when it happens.
In any case, the hijacker's game is up the moment they actually shoot anybody, as their zip gun is almost certainly difficult and slow to reload, and they will be trapped in close quarters with scores of angry and frightened passengers. With this in mind, they'll almost certainly have better luck producing realistic looking fake guns. A submachine gun, even a fake one, is more likely to scare passengers into cooperating, and that cooperation is what gives them a chance to succeed.
>"The most such an attacker could hope to accomplish [...]" //
I think you're lacking imagination there.
"If pressure was lost at that altitude, everyone aboard would have been incapacitated almost immediately. Autopsies will tell if they died before the crash or from the impact."
It seems like a rapid/explosive depressurisation event would be possible to create from inside the cabin with an explosive weapon. I'm not saying that's going to down the aircraft but it seems it would create more of an incident than your imagined scenario.
The pilots should have time to don oxygen masks unless pressure is lost very rapidly at very high altitude. I'm doubtful that just shooting the plane is an effective way to accomplish this.
Doesn't have to be a bullet. In the link (which is a small plane) a door/window seal is suspected as the cause of bringing down the plane. But surely if you can compromise a seal or something and cause a rapid decompression that's going to more than likely kill some passengers at ~12km cruising altitude.
Looked at other cites and there was a study on pilots showing that even a 2s delay from decompression to them fitting masks was significant in reduced control.
My point was only to challenge that the very worst you could do was cause a minor panic without affecting the mechanical flight at all or injuring more than maybe 1 or 2 people.
I didn't check but IIRC that Wikipedia article refers only a bullet passing through the fuselage? In which case the hole is too small to cause even rapid decompression.
