If a bullet were to penetrate a pressurized airplane, the passengers would not be sucked out the windows from "explosive decompression." That is a persistent urban myth originating with the 1964 movie, Goldfinger, starring Sean Connery as James Bond.
Airplanes already have holes. Air is constantly pumped into... and out of... the plane. (Otherwise, the passengers would suffocate.) It is not a closed system. The size of the hole (the "outflow valve") depends on the size of the plane, but it is a big hole. Outflow valves are over a square foot on the 737, up to two square feet on the 757, and so on. You can lose three windows and still keep the cabin pressurized. A 9mm/.357 caliber bullet makes a hole with an area of 1/10th of a square inch. (Area = pi R squared.) The effect of a bullet hole on cabin pressure is not enough to be measurable.
Explosive decompression only occurs with huge holes. In 1986, a bomb blew a 20-square-foot hole in a TWA 727 over Athens, and 4 passengers were killed. In 1988, an 18-foot section of the roof came off an Aloha Airlines 737 mid-flight, and one flight attendant was killed. (Both planes landed safely.)
If the Goldfinger Syndrome were true, the Airline Pilots Association would not have voted to arm pilots, and the FAA would not be talking about armed sky marshals.
It's a myth, OK? It was just a movie.