We’ve seen it in the movies — someone shoots a gun aboard an airliner and a vast, gaping hole opens up in the fuselage through which passengers are sucked out followed by seats ripped right out of the floor. It’s exciting to see in a James Bond film but it’s Phony Physics and no more realistic than Bond’s sportscar that turns into a submarine.
As an airliner ascends, cabin pressure is reduced from what it was on the ground to a pressure equivalent to that found at around 8000 feet altitude. At cruising altitude for an airliner, this will result in somewhere between 8 and 9 pounds per square inch of pressure differential between the cabin and the outside.
Would air get sucked out through a bullet-sized hole? Yes, but the explosive decompression so beloved by Hollywood action movie directors would not take place. The hole could probably be patched with tape or the leakage greatly slowed by plugging the hole with an appropriately-sized object. Even shooting a window would just result in, well, a hole in the window. At worst, the pilot would have to deploy the oxygen masks and descend to a lower altitude.
The danger of actually puncturing the fuselage if shots are fired can be reduced by the use of frangible bullets such as the Glaser Safety Slug (www.safetyslug.com/Ammo_Info.htm) which is designed to minimize penetration.