You may be talking about a "set gun". I have known people that rigged them up to kill gophers in their farmyards using an old single barrel shotgun and some trigger activator like a cord from a mousetrap mounted to trip when the gopher appears and snaps the mousetrap to fire the gun. I remember hearing older relatives talk about old timers setting them up to stop repeated burgleries but since there are a million legal reasons to not do it (like firemen & cops getting killed by them)they've been outlawed for many years to install one in your home or business. I don't know about "agricultural use" to control vermin.