Ken is quite right. If you have the room, IMO the better bet is to buy a vault door and construct your own vault of reinforced poured concrete, walls, ceiling, and floor, and install a nice dehumidifier system. I did, and wouldn't have anything else now. I bought a Browning vault door, there are others available as well. Finish the interior with carpeting on the floors, a commercial grade grey plain carpet on the walls (has a foam back and helps deaden any accidental blow of a barrel against the walls) and whatever you want on the ceiling. Add shelving, racks, suit yourself.
