I once bought a 16 gauge Fox which an owner had placed two paper empty hulls in the chambers as snap caps I think. Must have been there for decades. Over time those chambers rusted and pitted so badly that it was ruined. I had it chamber sleeved down to 20 gauge. In effect it became an over bored 20 with 16 gauge bores. Great shooting gun. After that I stopped using long term chamber mops or snap caps. For reasonable storage I’m sure they are great. Just periodically remove them and recharge them with a good oil.
