The above advice is spot on. Lots of old pistols have "box rash" because they have been stored in their boxes. I also cut narrow strips of clear document holders, and put them in the corners of the box where the muzzle, rear sight and grip base fit. I'm not sure if the rusting is caused by chemicals in the cardboard, or because the cardboard has absorbed some dampness from the air. In any case, it's best not to have any contact at all between the steel of the gun and the box.