Obviously this only works on domestic flights but the airline will not lose your luggage if they know they are dealing with a legally declared gun. If they do, they get sued by the ATF. Your luggage will be given first priority if you just throw a shitty cheap gun from Walmart in there.
deviant ollam has an old video about this, and remmeber that since american gun laws are kinda weird, a bent piece of metal could legally be a gun if you don't want to have what normal human would look at and call a gun