" legality of these gadgets " if you are referring to the ad, they were cap guns, legal where ever kids played, they were cap guns and not capable of being converted to fire real rounds, they were cast zinc. If you are talking real derringers in such a contraption, well, just as a lark, way back when Maverick was still a new show, I tried to carry a Remington derringer ( broken and non working of course ) in a hat, no way was it possible with out beating your head to a pulp. I do remember reading about a sleeve device, but they seemed to be awkward and unworkable except by Hollywood. As far as carrying a real gun in such a manner, the legality would have been the same as for any concealed weapon in your locale. However the hat trick only works in hockey.