With the cannon of the Phalanx Close-In Weapons System standing ready to deliver 75 20mm (.79 caliber) APDS tungsten rounds per second, not much:
Phalanx provides ships of the U.S. Navy with a "last-chance" defense against anti-ship missiles and littoral warfare threats that have penetrated other fleet defenses. Phalanx automatically detects, tracks and engages anti-air warfare threats such as anti-ship missiles and aircraft, while the Block 1B's man-in-the-loop system counters the emerging littoral warfare threat. This new threat includes small,high-speed surface craft, small terrorist aircraft, helicopters and surface mines. Phalanx accomplishes these engagements via an advanced search and track radar system integrated with a stabilized, forward looking infra-red (FLIR) detector. This integrated FLIR provides Phalanx with an unique multi-spectral detect and track capability for littoral warfare threats and dramatically improves the existing anti-air warfare capability. Block 1B also incorporates new Optimized Gun Barrels which provide improved barrel life, improved round dispersion and increased engagement ranges.Phalanx is the only deployed close-in weapon system capable of autonomously performing its own search, detect, evaluation, track, engage and kill assessment functions. Phalanx also can be integrated into existing Combat Systems to provide additonal sensor and fire-control capability.