Radar and gun in one mount
The R2D2-shaped white dome contains a Ku-band search radar and a Ka-band tracking radar mounted directly on the gun. There's no human in the loop and no separate fire-control computer in the ship — the system is self-contained. It searches, detects, classifies, tracks, opens fire and walks the rounds onto the target by watching where the tracers go.
Closed-loop spotting
The tracking radar watches both the incoming missile and the M61 Vulcan's outgoing rounds. It measures the miss distance pulse-by-pulse and steers the gun until the round stream and the missile track converge. This 'closed-loop spotting' is why a 20 mm round can hit a missile travelling 600 m/s.
Ammunition
Originally depleted-uranium penetrators, later replaced with tungsten for political reasons. Each round weighs 100 g and the gun fires 4,500 per minute — 75 per second. A typical engagement burst is 200 rounds, gone in under three seconds. The magazine holds about 1,500 rounds, enough for several engagements before reloading.
Limitations
Phalanx struggles with manoeuvring sea-skimmers (the Russian P-800 Oniks pops up and dives at the last second), saturation attacks (multiple missiles within seconds), and high-supersonic threats where the engagement window collapses below one second. The replacement, SeaRAM, swaps the gun for 11 Rolling Airframe missiles with longer reach and higher kill probability.