Excerpt:
the ammo used is no different than any other law enforcement ammunition. It will absolutely puncture the skin of the aircraft but a few small holes will not cause a depressurization issue. The air conditioning kits push more air into the airplane than you'd likely lose through a few bullet holes. If anything, the outflow valves would just close a bit more. Those valves let air out to prevent the pressure vessel of the aircraft from exceeding the max differential pressure (the ammo is not subsonic as another person suggested). The biggest danger is the possibility of other passengers being hit by stray rounds. However, when the assumption is that the bad guys intend to kill everyone on the plane, this is a small risk.