What weapon fires 20-mm, high-density penetrating projectiles at a rate of 3,000 rounds per minute?

The weapon that fires 20-mm, high-density penetrating projectiles at a rate of 3,000 rounds per minute is the Phalanx. The Phalanx is a close-in weapon system designed for point defense against anti-ship missiles and other aerial threats. It utilizes a rapid-fire Gatling-style gun, allowing it to deliver a high volume of fire in a short amount of time, making it effective for intercepting fast-moving targets.

The characteristics of the Phalanx, including its caliber and firing rate, make it well-suited for defensive operations in a naval environment, where the ability to respond quickly to threats is crucial. In contrast, the other options listed are different types of weapons or systems designed for distinct purposes, such as long-range strike capabilities or anti-ship operations, and do not match the specified specifications of firing 20-mm rounds at a high rate.