Minigun

The GAU-2/A (or M134) Minigun is a fixed gun used in aircraft. Firing at up to 6000 RPM from a single barrel and using the rest for loading and ejecting shells, it is an ideal weapon to fire while in the air to suppress enemies on the ground. The name is derived from the M61 Vulcan, a 20mm gatling gun used on jet fighters, with the minigun being a "miniaturized" version as it fires a small-arms round, 7.62x51mm NATO instead of an autocannon round. The Gatling design allows it to have a very high rate of fire without overheating.

Call of Duty 4
In Call of Duty 4, the Minigun is used in Heat to provide supporting fire to your retreating allies. The Minigun is attached to a downed helicopter (the same helicopter can be seen in the same position in Safehouse, but the minigun is not present) just past the church.

The Minigun can really do a lot of damage because of its high firing rate, but it needs to be spindled up and can overheat. However, keeping it revved will not overheat the gun, so you can keep it revved all the time.

It's also worth noting that the minigun is highly effective against the helicopters in the mission "Heat", it can normally down one in a small burst and will stop the troops getting off and engaging you later on in the mission. On Arcade Mode a helicopter kill is worth 1000 points. A quick spray around will down them all very quickly.

Call of Duty 4: Modern Warfare (DS)
The Minigun is constantly used throughout the Helicopter levels in the game, where you usually need it to eliminate opposition or take out objectives. Unlike its console counterparts it cannot overheat and does not need to be revved up before firing, making it an extremely effective weapon.

Call of Duty: Modern Warfare 2
The Minigun returns in MW2, but this time it is mounted on a HMMWV. The minigun is also used on most aircraft support, and on the sentry gun. It is used in the mission Team Player.

In Multiplayer, the Minigun appears in select levels, replacing the M249 as the standard "turret." This was most likely a balance issue, due to players in MW1 constantly going prone, shooting off a burst, then going prone again automatically due to the way the game handles it. Now, players have to expose themselves for the short spool time before being able to shoot.

Holding the left trigger zooms in and spools the gun barrels without shooting, reducing the "warm up" time significantly.

Trivia

 * In the game, the minigun is actually placed in the wrong opening. It should be placed in the "window" just behind the cockpit, just like the one you see on the friendly helicopter in Crew Expendable. However, due to the lack of the minigun in the mission prior, it could be assumed the SAS found it and moved it to a more useful position as they did with the Javelin missile in the church tower.
 * In the mission "Heat", Soap is ordered to lay down suppressive fire on the approaching enemies using the minigun. This is not possible as the minigun requires electricity, which in turn is generated by the chopper. The wreckage suggests that the minigun is useless. However if the minigun had been found, it may use a jury-rigged power source like a car battery.
 * In multiplayer mode, if you use a mounted minigun while wielding any sniper rifle, you can hold your breath while ADS with the minigun. It does not affect your accuracy, however.
 * In Homeland Security (a Special Ops mission), the sentry gun is a minigun.
 * In Of Their Own Accord, the minigun that Ramirez uses fires explosive ammunition, as evidenced by the small explosions and splash damage.
 * In Team Player if you fire the minigun while not being told to, Corporal Dunn will say "Allen, what are you firing at?! Cease fire! Cease fire!".
 * In multiplayer if you shoot you will not see your fingers on the trigger.
 * You can shoot the minigun without spooling it up in Call of Duty: Modern Warfare 2.