I would have to agree with Constable Clipcock - the risks and expectations of arming the flight crew are unacceptably high. To echo what I mentioned on another thread ...
As for firearms on the flight deck - hmmm, I'm not so sure about THAT. I've personally seen too many people exercise poor judgement with a firearm, particularly civvies and those who use them infrequently. I've seen recruits who have had drill after drill instilled in them and still screw-up on the firing range when under pressure or when the situation goes pear-shaped. Not to mention the risk of having ballistic weapons fire in a pressure hull. And what if you shot a first class passenger by accident (looking back from the cockpit) ...
Someone mentioned that a fuse breach through the skin could be acceptably controlled - true, but what if the round struck a window? I don't think they will hold up as well as aluminium somehow. What if the round struck something system critical in the wall cavity or floorspace? Besides there's not many things worse for passenger morale than turning the cabin into a rubber jungle.