ARMING PILOT'S?? This isn't the F****** Wild West!! I am an airline pilot, NOT a security guard, NOT a soldier, NOT a policeman. You forget that hand guns are ILLEGAL in the UK, on the ground, in the air, ANYWHERE.
I do NOT want the job of killing someone. If you must put guns on board, then with TRAINED sky marshalls who are trained to make a decision whether to kill someone without it being murder. Even then, having guns on board is just asking for trouble, all it takes is for him to be discovered, over-powered, and you now have armed terrorists. What if one of the marshalls is corrupt, a nutter, what if his family are taken hostage and he is told to hand over his weapon on his next flight etc etc.
I think you are forgetting the vast differences of opinion towards guns in the UK (and Europe) and the USA.
Mace, not such a bad idea, but the danger is
a) A good chance of knocking out both Captain and F/O in the comfined space of the cockpit unless both already on O2.
b) Also, in the event of a rapid decompression, a canister of mace might well explode or fracture - THE LAST THING you need at that moment.
The other problem is only one terrorist will be in the cockpit, the others down the back. You might be able to shoot one, but the rest will then start shooting back.