Loss of cabin pressure, as in what would happen if there were hole(s) in the airplane. We've planned ahead for holes in the plane for the past 40 years.
So go ahead boys, empty the magazine into those tangos and don't worry about little holes.
Better yet, practice good gun control - and get a tight shot grouping that doesn't damage the fuselage!!!
I would suggest you check out some of the resources pointed out elsewhere on this thread. It takes a heck of a lot more than just a bullet hole to cause cabin pressure to drop to an significant degree. Losing the airplane door would do it, but not a simple gunshot. Again, this is just hysteria generated by the "make sure they are all sheep" crowd.
"...don't worry about little holes."
Bob Kerrey has a word for the holes to worry about, or deal with - @$$holes. In this case, Middle Eastern @$$holes that poop islamofeces. It is time to make them deficate on our terms - scare it out of them with a well placed round.