A closed door to a safe, even a "breezy" stack-on safe, is going to prevent most of the circulated air coming in from the rest of the room. While not completely air tight, the closed door will do a decent job of preventing that ambient humidity from entering. A desiccant will dehumidify an stack-on safe quite well.
General guidelines: "Iron and steel will not rust if the air in contact with the surface has a relative humidity below 50% relative humidity (RH)."
According to the NRA:
50% humidity at 70 degrees is considered ideal "Arms Museum" storage conditions. Any dryer then that, and you will have problems with wood stocks drying out and shrinking / checking.
By yourself a good hygrometer and measure the humidity where your safe will be...and measure it often. That will tell you if you really need to be concerned with desiccant at all.
In my experience, rust prevention has more to do with regularly wiping down your guns to remove hand oils than it does with humidity control.
That said, I use desiccant because I live near to the ocean. I use "DampRid" which is cheap and readily available at most grocery and hardware stores. It comes in a variety of container sizes and shapes. It works great, but does need to be replenished every so often with new powder.
Another idea is to put a 4 or 7watt incandescent nightlight inside your safe. Works
exactly like a "Dri-Rod" dehumidifier...only a lot less expensive. Provided your safe isn't the size of a walk-in closet, the small amount of heat from the bulb is just enough to circulate the air and slightly raise the ambient temp inside your safe, which in turn lowers the relative humidity inside the safe.
I hope this helps....good luck!
