Fact: Most shotshells no longer have brass case heads. Rather these are "brass plated" steel case heads.
Recently, I discovered several five packs of various buckshot brands, all with brass plated steel case heads. These had been inavertently left in a closed carport storage room for over a year. This room had no heat or a/c in a humid "Deep South" environment.
Rust had formed on the shotshell case heads in every area in direct contact with the cardboard "five pack" boxes. Except one. That was a 5 pack of Rio Royal Buck. In that box, the cardboard interior had a plastic coating. No rust!