From what I understand about the propellant used in [i]all[/i] weapons is dust based, even if it doesn't have some sort of elemental effect.

Examples are in the first minute of the first episode of RWBY, and listed [url=http://rwby.wikia.com/wiki/Dust]here[/url] where it mentions that Ruby's bullets, despite having no innate elemental power, use Dust.