One of my customers is a MAJOR stock manufacturer. He says 55%humidity is target storage humidity for guns.
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If a dehumid unit is in your budget, that would definitely be the way to go, it would also allow you to keep the room at a moderate temp while maintaining the humidity level you want. It goes without saying, if the room is warmer (70 degrees) then you will see much less issue with condensation on your weapons when/if you take them out of the room.