Toprotectandserve -
Hey, I think every FFDO in the U.S. is in agreement with you about no gunfire onboard aircraft. They do like having that option should the foolproof
protective layovers ever fail. When needed I think they'll overcome any desire to preserve their hearing and willing trade potential hearing damage in favor of maintaining control of the aircraft in a desperate situation.
You mention 'specfic threat'. Since civil aviation, especially involving U.S. carriers, is targeted that's enough of a specific threat for the authorities in the U.S. to authorize the FFDO program.
The attempt at maintaining aircraft security is multi-faceted. Obviously there's disagree about what is, or isn't, reasonable policy. In the U.S., and several other countries, the side in favor of more armed individuals has the upper hand after debate was heard from both sides of the arguement.