I think piloting a plane and possessing a firearm require two completly different mind sets. The police don't even let all their officers carry them so why should pilots?
A question I asked on another forum which never got answered when people were talking about strengthening flight deck doors and arming crew is that the crew were lured off the flight deck by the terrorists attacking cabin crew and passengers. Having a firearm is not going to help in this situtation, especially if they are holding one of your crew with a knife to their throat!
Julian.