Thomas, a single bullet is unlikely to depressurize the aircraft: the pressurisation system works on the principle of controlled leaks, rather than sealing the air in: the outflow valves would close a little, the cabin pressure wouldn't change much (although passengers seated near the hole might notice a whistling noise.
I was fascinated by your assertion that a 9mm has more stopping power than a .44": nope! Sure, the 9mm may have a higher muzzle velocity, but the considerably heavier bullet fired by the .44 makes up for that. And if the hijacker also has a firearm? Well, you're not really any worse off, are you?