[Cronos]

If a terrorist hijacks a train -- use a guna nd blow his brains out, no worries. Use a gun on a plane, blow the t's braisn out and you may shatter a window, causing the air to get suckeed out, maybe ripping the fuselage (if the aircraft is pretty old and the metal is badly stressed), killing those folks you were supposed to save.

[COEXERJ145]

If a bullet managed to penetrate the aircraft skin or a window, yes the aircraft would decompress but that is why every aircraft has oxygen masks. What we see in the movies where a 9mm causes a chunk 5 seat rows long to break off is not very likely. Perhaps something like a .44 or .50 magnum to just the right structural joint might do major damage but those guns are just a bit hard to conceal.

Also, I'm pretty sure the bullets the sky marshals and pilots carry in their gun are frangible and designed not to penetrate anything except soft targets such as a human body.