Phalanx!
Yes I think it would be easy to refit the Phalanx CIWS to a smaller Gatling gun that would shoot relatively cheap shotgun shells and fill the sky with shot.
“Phalanx”!
You beat me to it.
Phalanx!
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Didn’t work shells to big, gun action set for fast moving targets not slow ones.
Suggested in article comments is mini-cannon using .30 cal rounds instead, as well as some sort of weapon firing shot gun-like round (WWII naval AA gun?).
Wait until the Navy starts mounting lasers on DDG’s. Maybe they’ll bag a few aliens?