One gun and many purposes? My first thought was for a combination weapon, like a drilling. I have seen European triple barreled weapons with three different bores. Small bore rifle, large bore rifle and shotgun would be a great choice. Most people have never seen this sort of weapon, so I voted for a pistol. I have a .22 cal kit for my 1911, along with the stock .45 acp. Other calibers could be worked into the area of operation. Fashion a rifle type stock that either attached to the pistol, or worked in concert with the shooter to stabilize the weapon and range is increased.
Shot gun ammo is to large and bulky and shotguns are hard to hide or concel.
Shot gun ammo is to large and bulky and shotguns are hard to hide or concel.