I am a maintainer / operator of the Mk 45 5'/54 cal Gun mount and I can tell you that it is extremely effective in the AA role. The HEVT or High Explosive Variable Time round has a Doppler radar in the fuse which detects the aircraft/missile and detonates the round creating a wall of shrapnel several meters wide which destroys the target when it fly’s through it.I didn't think the 127mm (US mount) did have an AA capability? Also isn't the USN planning to use 57mm guns on the new DD1000 as CIWS and the French/Italians have just installed 76mm on there new AWD as CIWS. Phalanx is wonderfully portable/bolt on but does it have the range/hitting power for faster/heavier AShM?
For a navy that wants to operate with some independence but does not have the money for AWD, carriers or even land based jets, a robust layered defence is important. I would go for 57mm, ESSM and Goalkeaper or Millennium 35 mm.
On the old MK 42 5"/54Cal gun mount (as found on Perth class DDG's) we used to use VTNF or Variable Time Non Fragmenting round for practice AA shoots. This had the same radar fuse but a was a low explosive round as to not damage the target. However when the Mk 45 was introduced when the ANZAC's where brought online we very quickly stopped using VTNF rounds for practice AA shoots as even the low explosive rounds where knocking the targets out of the sky.
We now use Ballistic (Inert) rounds for the AA shoots and the targets often get recovered with the blue paint of the Ballistic rounds scraped down the side, which shows the phenomenal accuracy of the MK 45.
The MK 45 is really the only "All Rounder" out there that can do it all (with the exception of the OTO Melera 5" gun) . One gun to do NGS, AA and surface engagements!
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