I am working on a series set a few hundred years in the future where fusion power is common and starships roam between planets. I am trying to think of a way to power small Arms weapons and keep them feasible, so no laser rifles or coil guns due to a lack of power storage and heat issues. (unless there is a plausible battery that could be developed to allow for a few hundred rounds (1000kJ ) and have be less than a kg).
Back to the point, my main idea is to stick with chemical propelled projectiles such as bullets (caselsss ammunition).
The question: Is there a potential chemical/energy source that could be attached to the bullet that would be more compact and lightweight (and have the same output) as a traditional modern charge?
The ability to fire in space is also a prerequisite.