I've actually tried launching airsoft bbs with 60 joules, and the results weren't very impressive. fine aluminum powder should work as a propellant, but you'll only need a very small amount. I think lead would be a fairy idea metal, but unfortunately lead vapor is toxic.
Something I didn't think about until just now: If your propellant gasses are 10,000+ F, the particle speed is going to be ridiculously high. When gasses reach such high temperatures, they don't seem to mind choked flow* very much. But, for people using them to launch projectiles, this means that any small gap around the projectile will let a lot of propellant gas escape compared to, say, a room-temp pneumatic.
* if you don't believe me, try playing around with disk diameters in hgdt.