At the risk of repeating something posted a year or wo ago on pprune;
I had to chair a discussion with a group of an Arab carrier's flight deck aircrew in 2000 on the subject of airborne security and prevention of hijacking.
The irony was that several of the people in the room were past practitioners, reformed into law-abiding aircrew.
The single conclusion that they came to, with no-one dissenting, was that the ONLY way to ensure that airborne piracy fails is to have an impenetrable barrier between cabin and flight deck and never to open it while the engines are running, FOR ANY REASON WHATSOEVER.
Thus there needs to be facilities for peeing or worse, food and water, and an ability on the part of the crew to ignore whatever is happening in the cabin, including murder, while they get to an airfield for landing, land, and wait until the authorities arrest or kill the terrorists.
Quite a tall order, really, but they were absolutely right. They knew, as we all do, that an aircraft can only be taken over by gaining control of the flight deck. Armed pilots may kill or injure a terrorist, but are more likely to be killed themselves before they achieve that. If an armed and fanatical terrorist breaks into a flight deck he or she can probably shoot the people there before they even begin to react. If the terrorist is slowed down or prevented by the barrier, then that proves the point about the barrier being the key.
Arming pilots achieves very little. Arming cabin staff would be a lot more effective.
EDIT: But please don't even begin to think of arming cabin staff; I just said that to make the point!