A Phalanx Close-In Weapons system (CIWS) during a live-fire practice by a guided-missile cruiser in November 2023. MC2 Malachi Lakey/US Navy

The War Zone has a good article on the Phalanx weapon system that arms US Coast Guard National Security Cutters (NSC) talking about ammunition costs.

The cost of ammunition for these systems actually looks trivial compared to the cost of missiles, but I am not really a fan of their use on cutters.

While it has been improved, the basic system goes back to 1973, plenty of time for adversaries to redesign their missiles to minimize its effectiveness.

The projectile is a high density, solid 12.7mm (.50″) sub-caliber discarding sabot round with no explosive content.

An enemy using anti-ship cruise missiles will endeavor to fire several missiles and have them arrive simultaneously. On a Navy DDG or FFG, they can begin countering cruise missiles as soon as they appear over the horizon or even earlier, as demonstrated recently in the Red Sea. Consequently, their Phalanx would only have to deal with rare leakers.

Cutters have, at best, a chance of bringing down a cruise missile with the 57mm. We don’t yet have a smart munition considered reliably effective against cruise missiles.

Phalanx’s effective range is reportedly 1,625 yards. A 600 knot anti-ship cruise missile covers that distance in less than five seconds. New generation supersonic missile cover that distance in far less time. The system does not move on to a second threat until the system’s radar recognizes that the first target is no longer a threat. It seems unlikely that Phalanx could engage more than one missile, much less more than two if they are timed to arrive simultaneously. There is also a good possibility that even if successfully engaged at very short range, missile debris might still impact the ship.

Replacing the Phalanx with SeaRAM, which has an operational range of 9 km (5.6 mi), would at least allow it to engage several targets simultaneously since it is a “fire and forget” system. The switch should be easy. The mount, footprint, and support requirements are the same with minimal changes required to the ship. Like Phalanx, SeaRAM is an autonomous system. It can also be used against surface targets.

Even with their superior long-range systems, the Navy began replacing one of the two Phalanx systems on some destroyers with SeaRAM in 2015. The Littoral Combat Ships which have a combat system similar to the National Security Cutters were equipped with SeaRAM or RAM from the beginning (2008).

There are no US Navy ships armed with only guns and CIWS.