Condensation usually happens when you bring a mass of cold metal into a warm room. Pull a "cold one" from the refrigerator and set it on the counter. You'll see what I mean. It would be less normal to have condensation on a piece of warm metal. You can put your airgun into a plastic bag or gun case outside then bring it in and allow it to cool to room temperature before removing it but that would probably defeat your purpose as the cooler conditioned air inside the house will probably be drier than the warm outside air.