I really don't care if the person carrying is legal or not -- I don't want guns on a pressurized aircraft unless the person carrying them has been SPECIFICALLY trained for that situation. Passing out special ammo at checkpoints is bloody useless unless you load the weapon for them -- and how exactly are we to be sure that we have the correct caliber for all these weapons?
Disaster is assured when Deputy Barney takes that ammo and fires directly at a passenger in a window seat at point-blank range. Even that ammo will shatter a window if you hit it right.
Now if you want to turn the WC on the aircraft into an explosives sniffer (complete with a retardant release system), then maybe we can talk. If the alarm goes off and you don't respond to the FA, then you take a bath. (And we can vent the WC, too, so that whoever is in there loses consciousness from lack of oxygen.) That plan is at least as good as arming Deputy Barney.