Well I know some about weapons and that kind of weapon require quite a lot of energy to feed up. So I dont think of any mechanism or system that can do that in such small scale of cuorse if you have LN within the rifle and some magnets capable for carrying tons of metal with just little currents it can be done. Also I dont have much information about W40K universe. (Actually in one of my games sniper class uses same type of weapon but its quite big and hard to carry even with an enchanted battle suit)