This may be a repeat, but I also have a shed for my reloading, here's what I did. Insulate the walls and made sure that there was fresh air moving from the soffit to the peak of the roof in the summer that helps to vent the heat. My building is on the small side so I bought a window A/C that was oversized (to allow for it not being as tight as a house) and had a "eco" setting. On mine it samples the air every minute or so and only runs the compressor when needed, it made little difference on the electric bill. I also put in a de-humidifier that drains outside, on cooler days it keeps it dry and adds some heat. And because I'm anal I put silica packets in any open powder and also put the silica in the drawer with the primers. In 2 years I've never had an issue with anything rusting or bad powder/primers.