The storm rifle is a fully automatic plasma weapon that was invented and used by the storm covenant, its main advantage being it isn't accurate enough to actually hit a enemy unless your close enough to take a shotgun blast anyway, and that compared to the simple plasma rifle it sucks, and the disadvantages being basically everything else about it.
The storm rifle like many covenant weapons fires plasma that is powered by a forerunner tec energy core. It may not be able to hit a target but on the bright side even a near miss can cause a 3rd degree burn, if some one some how actually manages to score a direct hit it may even result in dismembering a arm or a leg. Once it runs out of power it can be recharged.