It's All About Free Water Wash tank or, as it is more commonly called, a gun barrel–is a settling tank fitted with an internal or external boot, or flume. Although gun barrels are not employed to the extent now that they once were, many are still used to break emulsions. In a gun barrel, oil and water are separated in a two-part action: (1) washing, and (2) settling. The washing is done in the free-water layer, and the settling occurs in the emulsion layer. Because all emulsi