Penguin
What's with the Rambo mentality? You do realise what happens when an aircraft is suddenly depressurized right?
*ahem*
Check Mythbusters:
They did an experiment where they fired a bullet into the wall of a plane (pressurized as if it were at 35,000 feet or so), and it did not suck everything out.
Next, not everyone has the guts to kill a person, you're looking at maybe on or two people firing. Third, you would have to get through quite a few bullets before you actually would be looking at problem; note that the guns are all pistol caliber. If you want to go down to 9mm, that works too. The Glock 18 (full auto) and 17 (semi-auto) provide great firepower for the crew and passengers, respectively.
This is also why those masks drop down, to give you oxygen while the crew goes down to lower altitudes: how do you think that those bomber crews in WWII survived? Finally, anything that the passengers do to themselves is better than what the hijacker will do: either blow up the plane or fly it into something.
-Penguin