You have got to be kidding. How do they get it if they need it in a hurry?
They shouldn’t need it in a hurry. The cockpit doors are strongly reinforced and always closed and locked now during flight.
The theory must be that at the most vulnerable moment, when the door is opened for the pilot to exit, or enter for that matter since in most cases the second pilot will not be a FFDO and thus not authorized to unlock the weapon, or have his own, that's when the gun should be locked up.
Obviously the federal authorities are more worried about the pilot taking the gun into the cabin, or letting the guy with the lives of the passengers in his hands, the other pilot, having access to the firearm, without "official blessing", than they are about terrorists taking over the aircraft.