It's kinda funny you mentioned that. Just last week, me and some friends over on the other site , where brainstorming a two stage bullet, that came to me in a dream.
went on for awhile, until I found that NASA had kind of the same idea I had, but had applied it to a gun instead.
It was basically, Gunpowder hits piston, which compresses hydrogen, which propels projectile at mach 8.
Stephen