Okay, let me try again. I did not say rust, I'm not talking about rust. I'm taking about what happens when stainless steel barrels touch the aluminum hull of the boat in the presence of salt laden moisture, be it the air itself in close proximity to the sea, or the saltwater spray itself. While the barrel might get rust specks in places if left in the boat for a day or two, it is the the permanent 'greying' of contact points with the boat I am talking about, and they can happen in a very short time. No amount of rubbing with steel wool and bore cleaner- at least nothing that doesn't also scratch the barrel's surface- will remove it. If it were streaks from the softer aluminum I think it would be removed by the steel wool. It isn't. It also doesn't appear to be or feel like pitting. Perhaps it is a thin aluminum oxide transfer? (But that would involve some kind of a chemical change.)
This happens regularly. And these kinds of metal interactions are definitely something I am not unfamiliar with. Hardly a metal is unaffected in our coastal conditions. Dissimilar metals most definitely interact in the presence of saltwater. A boat with the electrical systems associated with an engine added to the mix most certainly create more opportunity for (negative) metallic interaction.
