The chamber rusting with plastic shells was not confined to Superposed guns. Recall reading a report on this many years ago in the Rifleman. Seems the problem was the chambers are left dry & unprotected after firing. The wax treatment on the paper hulls left a protective coating in the chambers after they were fired. If the chambers are not given some protection shortly after firing they are left in a condition conducive to rusting when plastic shells are used especially if enough shots have been fired to build up heat enough to dry out any oily deposit which may have been there.
