I am fairly new at this gun ownership business, but I do know a bit about storing valuable things safely.
I know antique textiles, table linens, clothing, quilts. A preservationist would NEVER STORE such items in ordinary cardboard boxes as part of the cardboard/paper making process the boxes have a form of acid in them. Over time, items stored in cardboard can turn color and even rot from contact with the cardboard. For the same reason, fine artwork is always framed using acid free mat board and textile archivists use acid free tissue and acid free boxes to store textiles. The acids present in the cardboard boxes can over time have a detrimental effect on metal finishes and finishes on fine wood also. For that reason I would not store firearms in boxes, unless they were the acid free archival type.
Wool holds moisture therefore any case for a gun that contains real sheep fleece is not advisable for gun storage. Many makers of fine leather gun cases line them with synthetic fleece so they won't attract moisture. As others have said, the zippered gun cases, especially leather ones are not good for long term storage as they would have the potential to hold any moisture that would be in the environment.
Cotton, on the other hand, such as the mention of the washcloths used to separate pistols in the walmart pot lid racks (that's what they are) is an excellent idea as is breathes and doesn't hold moisture like wool does. Cotton dries faster.
I have my small, very small, group of handguns slipped into white cotton socks. One per sock. Then they go in the safe with dessicant packets. Get a bag of Cat Litter Crystals. Just the crystals, not the clay litter with crystals. Take some old panty hose, guys, your wives/girlfriends have plenty. Cut each leg of the hose into 6 inch pieces...you will have little 6" tubes. Tie a knot in one end then pour about 1/2 full of the cat litter crystals and tie off yhe other end. Voila, you have dessicant packets. You can make the packets any size you need. The nylon is indestructable and won't rot. If you need to "renew" the packets ability to absorb moisture set them in a warm dry place for the day, I put mine out in the sun in the summer and you could put them over a heat register to dry out in the winter.
The Ruger gun boxes that I have are stacked inside one of those vacuum storage bags that you store blankets,etc in, then you vacuum out the air. That keeps the boxes in good shape and protected from dirt and any moisture in the basement. They will pristine for the next owner of the guns, should I ever sell them. You can fit a LOT of cardboard or plastic pistol boxes in one of those giant vacuum bags.
You all have some very good ideas about storage. Thank you for additional information that I did not have before. I always appreciate what I learn here.
Viking Queen