I kind of doubt that they would use brass casings stills, a lighter and stronger material would allow you to carry more shells, and better heat conductive properties will keep the system cooler. Though I personally wouldn't be surprised if they used combustible bag charges, but that would be the caseless ammunition...
As to what propellant is used it would also be some future material, my money would be on it being a solid material most certainly due to the higher energy potential for size you get with that. But as to combining two chemicals together to fire projectiles? If you consider plasma a chemical than I can imagine it being plasma ignited propellants, an idea that has been floating around for a while now as the next step in tank armament, since rail guns would require nuclear powerplants for now.
That reminds me, are there still rules for caseless ammo, or has that been dropped?