T. O'Heir said:
Desiccant packs literally wear out by getting saturated and require periodic replacement. They don't do much in a regular non-air tight case anyway.
But these are Pelican cases, which are airtight, and desiccants can actually do too much in them or in any other sealed case. If the desiccant has adequate capacity, it will dry out everything inside the sealed volume and will not wear out because no additional water comes in to replace what it picked up.
The problem in this situation is they can lower the moisture content of wood stocks to the point they start cracking. I don't know of any problem with synthetic stocked guns, but wood and wood finishes can be damaged, so I would watch out on that score. You only need to get relative humidity below about 68% to prevent most rust. A heater, like the Golden Rod heaters for gun safes, will do a good job of this but are too big for a Pelican case. What you can do, instead, is if you have a cool area like a basement, but one that's not too humid, you can let the guns and cases settle at that temperature, seal them and then store them in a warmer area than that.
Note that desiccants can get some organic vapors as well, so if you apply a protective coating that can be worn out by gradual evaporation, be aware the desiccant can accelerate that wear. Putting VPN in with a desiccant, for example, can use them up faster.
What heating does is increase the ability of the air to dissolve and hold water vapor. This doesn't reduce water content in the container, but it does increase how much of that water is held by the air, thereby reducing the amount in everything else in the box. In layman's terms, it's called drying by applying heat. But also, by holding on to the water harder, the warmer air makes it unavailable to promote rust.