While I couldn’t give you the exact number, I do know that the Phalanx 100% kill distance is nowhere near 8 miles. 0.8 miles would be a much better estimation-it’s a 20mm non-explosive round. Another thing to remember is that I don’t know if all USN Phalanx mounts have received the anti-surface block update yet. The mount in your photo hasn’t.
I’m sure you know way more about it than I do. Let’s just hope they shred a lot of Muzzies when needed.