The whole issue of a loaded firearm on an aircraft, in the hands of even a trained professional "marshall" is foolish, especially in the flight deck. There is nothing to stop a flight crew member from beating a weaker flight crew member senseless then turning an aircraft into a much larger more powerfull weapon than a firearm anyways.
A bunch of idiots flying around with a false sense of security. Don't get me wrong, I like fire arms. I see much more of a need to carry though in a poor neighborhood than on an aircraft. The part of the country I live in most home invasions happen between 10am and 2pm for the simple fact that just about everyone has some form of a firearm in their home.
Now there are other effective weapons that may be more suitable for the cockpit if you wish to apply the same mentality of scaring the flying public into submission.