I have one set of snap caps that contain VPCI, or Vapor Phase Corrosion Inhibitor. They seemed to work well. I also wrap guns in VPCI plastic for longer term storage. Placing some in the gun safe adds an extra layer of protection. VPCI products emit a vapor that displaces atmospheric oxygen from metal surfaces, and keeps steel from rusting.
A piece of VPCI paper or plastic also seems to work very well to prevent rusting of tools in my roll-around tool boxes in my shop, since it is not heated when I'm not actually in there working. I just lay some in the drawers under my tools. Prior to using the VPCI materials, sometimes condensation would form on cold metal surfaces when humidity levels rose and temperatures dropped. Here's a link to the company that produces VPCI products. It is often found used to wrap bearings, gears, tooling, and precision parts that need to be protected from corrosion during storage and shipping. They say it only lasts for a couple years, but it seems to keep working longer than that.
https://www.theruststore.com/Learn-about-VCI-W23C2.aspx
