Timing is even more critical with a single missile. Multiple barrels with stacked caseless cartridges that are shot out over a few milliseconds. That should cover a couple hundred yards.
...until you find out that in such a situation the Metalstorm-like box will only have some of the first layer of rounds go even vaguely towards the target as the recoil will push it out of line. Remember, it’s only attached to a missile.