With proper training a firearm WILL NOT discharge unless you want it to. Beats the !!!!! out of having my throat slit and my 727 turned into a cruise missle. Maybe you Europeans don't like guns, but I'm from New Hampshire. Everybody's got a gun. I can't remeber the last time one accidentaly discharged. As for deterrence, our crime rate is unbelievably low.
If you don't want a firearm on your flight deck ;fine. I want one on mine.