Toy rocket inspires variable-speed bullets - tech - 21 July 2008 - New Scientist Tech
I'm not sure what I think about this. On one hand it is alluded in the article that varying the projectile speed would depend on using different types of bullets to achieve the variances....which in my mind doesn't really gain you anything. Not very efficient if you have to change out mags or the like to get the desired effect.
But it does mention that the projectiles would be fired using a propellant detonated in a combustion chamber, which I think is essentially a variant of "caseless" ammunition. I'd like to think that someday we will be going this way with firearms technology if they can work out he bugs that are associated with it.