MetalStorm is essentially a way to deliver ALL your rounds in a weapon onto the target at almost the same time. Still unproven and undeployed.
Rather than having a magazine or a belt feeding rounds into a breech in a manner that hasn't changed that much since the first machine guns, all the rounds for a Metalstorm are stored in the barrels. The rounds are triggered electronically and leave the barrel sequentially. When the barrel is empty you replace the barrel.
So no moving parts (other than the rounds), no shell casings, recoil effect on the second round limited as it all happens so fast, massive firepower brought to bear very quickly and no reloading magazines as the barrels come already loaded.
Some movies of this new concept in perforating your fellow man are over at MetalStorm Ltd