I think fours ought to be imported as religious artifacts. I for one, would say a few prayers before pulling the trigger, and perhaps a Hail Mary or mutter a Sweet Jesus afterwards. Perhaps you could get around the bore size if it identifies as a slug gun instead of a shot gun. One of the famous ivory hunters used a four and his main complaint was it gave him nose bleed every time he shot it. Recoil for that cannon was brutal. But that was a single shot not a double.