The 20mm/99 M61 Vulcan hydraulically/pneumatically-driven, electrically-fired, air-cooled, 20mm six-barrelled rotary cannon fires at an extremely high rate of between 4000 to 6000 rounds per minute. The Vulcan has been the main cannon armament of United States military combat aircraft for over fifty years. The several aircraft at the time were originally designed without an internal cannon, as it was believed that missiles had made guns obsolete; however, air-to-air combat experience in Vietnam showed that a gun could still be more effective than guided missiles in many combat situations, and that an externally carried gun pod was less effective than an internal gun, so the M61A1 Vulcan became standard as of the F-4E Phantom II. The initial M61 used linked, belted ammunition, but the ejection of spent links created considerable problems, and was soon replaced by the improved M61A1 with a linkless feed system. A lighter version of the Vulcan, the M61A2, was developed for use on the F-22 Raptor, which is mechanically the same as the M61A1 but with thinner barrels to reduce overall weight to 93 kilograms (202 pounds), allowing it to reach a slightly higher maximum rate of fire at 6600 rounds per minute. In addition to being mounted on the majority of combat aircraft, there also exist two land-based applications: the M163 Vulcan Air Defense System self-propelled anti-aircraft gun and the M167 Vulcan Air Defense System towed anti-aircraft gun.

The Mark 15 Phalanx CIWS is a close-in weapon system designed to engage anti-ship cruise missiles and fixed-wing aircraft at short range. The Phalanx is the most numerous CIWS in the world and has been exported to many other countries. Like other close-in weapon systems, Phalanx provides ships with a terminal defense against anti-ship missiles that have penetrated other fleet defenses. Consisting of a 20mm M61A1 Vulcan and a Ku-band radome mounted on a swiveling base, the Phalanx has been used by multiple navies around the world, most notably every class of surface combat ship registered by the United States Navy, as well as the navies of 19 other nations. As an entirely self-contained unit capable of automatically searching for, detecting, tracking, engaging, and confirming kills using its computer-controlled radar system, the Phalanx is ideal for both fully-equipped combat ships and support ships lacking integrated targeting and sensor systems. The Japanese Navy also fields a smaller, more basic form of the Phalanx simply called the 20mm/76 Gatling Gun for mounting on patrol boats too small to carry a full-sized Phalanx. A land-based variant also exists—in response to constant artillery attacks on bases in Iraq, the United States Army ordered the development of a quick-to-field anti-artillery defense system in the same vein as the original M167 VADS as part of its Counter-Rocket, Artillery, Mortar (Counter-RAM) initiative. The end result was the Centurion system, each consisting of a modified Phalanx CIWS, armed with a 20mm M61A1 Gatling gun capable of firing 4500 rounds per minute, powered by an attached generator and mounted on a tractor trailer for mobility.

The M197 Electric Cannon is a three-barreled electric rotary cannon developed primarily for use by United States Army helicopter gunships. The M197 is essentially a lightened version of the M61 Vulcan cannon, with three barrels instead of six. Its maximum rate of fire is limited to one quarter that of the Vulcan at 1500 rounds per minute to reduce recoil to acceptable levels for light aircraft and helicopter use. The M197 went into service on later marks of the AH-1 Cobra, using either the M97 or A/A49E-7 armament subsystems, and was also fitted in a ventral turret on the USMC YOV-10D Bronco NOGS. It is also the basis of the GPU-2/A gun pod, which incorporates the cannon, a battery and electric drive motor, and 300 rounds of linkless ammunition.

Description [partially] lifted from Wikipedia.

Powers and Stats

Tier: 9-B single shot, higher in short bursts

Name: M61(A1/A2) Vulcan | Mark 15 Phalanx CIWS | M197 Electric

Origin: Real World

Age: 1959–present | 1980–present | 1967–present

Classification: Rotary Autocannon, Aircraft Gun, Anti-Aircraft Gun | Close-In Weapon System | Helicopter Gun

Attack Potency: Wall level single shot (about 62.7 kilojoules), higher in short bursts (4000–6000 rounds per minute rate of fire) | Wall level single shot (about 42.5 kilojoules), higher in short bursts (730–1500 rounds per minute rate of fire)

Speed: Supersonic+ (980–1120 meters per second)

Durability: At least Street level, at most Wall level in terms of total destruction (comprised of up to 112 kilograms of steel with a fragmentation energy of 2.96 megajoules) | At least Wall level, at most Small Building level in Vulcan Phalanx Mark 72 turret in terms of total destruction (comprised of up to 6200 kilograms of steel with a fragmentation energy of 165 megajoules)

Stamina: Barrel life of 9000 rounds

Range: Up to 5.5 kilometers (AA ceiling of 3.5 kilometers) | Up to 5.5 kilometers, 1.5 kilometers effective

Weaknesses: The combined bulk of the weapon, feed system, and ammunition drum can make it difficult to fit into a small, densely-packed airframe | The Phalanx is not as effective against modern missile threats; the expected real-world missile kill-distance is about 500 meters or less, so even if the missile is hit and damaged, it may not be enough to prevent the missile (or fragments) from hitting its intended target, meaning it could still cause damage to the ship's equipment and/or crew | The original linkless feed system had a jam rate of up to 30% (this was fixed in 2011), the recoil from extended automatic fire makes it extremely difficult for attack helicopters to maintain stable hover

Key: M61 Vulcan | Mark 15 Phalanx CIWS | M197 Electric

Note: Similar guns of about the same bore (20mm) and period generally have performance equivalent to this one.