>...only a handful of cases in which ships equipped with Phalanx have been the subject of missile fire, and none that have actually seen the system tested.
Worth noting that this line is now out of date. Allegedly they've been tested in Yemen recently [1].
tldr: the Missouri fired a chaff and the other ships CIWS was all “not in my airspace” and started shooting at it even though the Missouri was behind the target. Interesting, I wonder if the whole battlegroup has synced systems now to avoid such things, e.g. 1) I’m going to launch something, everyone else disregard it as a threat and 2) if a threat has a friendly behind it, don’t shoot it.
Maybe 2) is more of a judgement call since a few bullet holes is probably preferential to the alternative.
I don’t disagree, but I’m sure they had to come up with some mitigation after this. If each ship is shooting down the other ships primary/secondary/whatever countermeasure then in a worst case scenario you and your sister ships are only left with your CIWS. And they might still be busy shooting at your sister ships chaffs to deal with the real threat. I don’t know much about it, I just found it interesting.
Seems a bit like Star Trek - “Their shields go down for a split second when they fire”. Maybe the system goes dark for a second to avoid shooting down outgoing items? Syncing the shields might make this better (or expose other weaknesses?)
Phalanx is the US CIWS based on the 20mm Vulcan gun. in other words, Phalanx is the “last line of defense machine guns” to which you are referring.
You may be thinking of Aegis, the integrated combat control system. We don’t yet know why the Gravely was unable to intercept the missile further out. Could have been human error, could have been a sea skimmer getting too close before being detected. To date, Aegis has been extremely effective against ballistic missiles and old cruise missiles. But given enough time an enemy can test your potential weak spots.