Most soaps have salt as an ingredient. So that is bad. Simple green does not have salt, so it is a wise choice.
Balistrol cleaner was invented for cleaning blackpowder when militarizes still used blaclpowder. It can be used as a cleaner mixed with water, and a preservative oil straight out of the bottle. It has a basic PH by design, so it neutralized the acid in the black powder.
So salt and sulfur are big corroding agents, so try solutions to minimize their activity in your barrel.
