So, let's get this straight. A kerfuffle happens on a transatlantic flight, for arguments sake about 4 hours from the nearest landing field. A weapon is fired and the bullet hits a window. As a result of decompression and a limited oxygen supply the aircraft is forced down to about FL140. Fuel consumption goes through the roof and the aircraft is now at severe risk of not reaching any landable destination - the alternative is to subject 300 pax to hypoxia and probably lose a few.
Now there are non-lethal weapons about, such as CS and pepper spray, baton-rounds, and various toys beloved of martial arts practitioners and police forces. If we've got to have somebody armed on board an aeroplane, for goodness sake arm them with something (non-lethal) capable of incapacitating somebody, but not of penetrating a window or aircraft skin (or if it gets into the wrong hands forcing open a cockpit door). If worried about firearms being used by terrorists put yer copper in lightweight body armour.
With intelligence some sense could be made of it. But if we are simply looking at firearms-trained policemen armed with conventional firearms, it's a disaster waiting to happen.
G