Don't ever use leather for long term storage in contact with metal. It's not just the acids and tannins, but leather is hygroscopic, and will absorb and retain moisture from the air. Bad news.
I'm relining some trunk cases, and found some polyester batting at a fabric shop that looks like a good liner for barrel slots in a safe. Always white, but comes in various thicknesses. The thicker stuff offers some light padding and the polyester will not absorb or hold moisture. The silicone sacks are a very good idea, but if you don't want silicone, you can get untreated cotton gun sleeves from Cheaper Than Dirt, in 6-packs.
