This seems to miss the point. If there is a reasonable expectation that an obvious and easily obtained weapon like a gun or a large knife is going to be detected, then intentional attackers are forced to chose other targets or resort to more exotic and harder to obtain weapons.
In fact, as Boing Boing has already reported, the introducing metal detectors and X-ray machines resulted in a massive reduction in the number hijackings:
That means most of the guns and knifes seized are going to be from forgetful idiots. I am sure said idiots were subjected to some uncomfortable questioning and had their backgrounds checked before they were released, sans weapons. This would appear to be the system working as intended.
