METAL STORM type weapons on artillery and on large planes that are based, and can take off of artificial islands off the coasts. Cheap and effective.
Metalstorm ran into the problem that in as implemented form, it had major problems like the fact that each tube was basically an inaccurate single use weapon. Think about ballistics from variable length barrels.
Other problems that came out in testing:
1. Misfires tended to make the whole thing explode.
2. The infantry area denial system turned out to be at *best* as effective as a Claymore mine, while costing a couple orders of magnitude more than a Claymore, being heavier than a Claymore, being far more complex than a Claymore and being far less reliable. And larger/easier to spot.
3. CIWS application testing showed that a Metalstorm unit was less accurate, less able to switch targets, took longer to get steel on target and actually worse at trying to handle a saturation attack at any significant distance than either Phalanx or Goalkeeper, even given the same radar and mount.
This is why other than units for testing, no military ever bought a Metalstorm system. I can see uses for it, but not in this application.