Minigun

The GAU-2/A (or M134) minigun is a fixed gun used in aircraft. Firing at up to 6000 RPM from six barrels at once, it is an ideal weapon to fire while in the air to suppress enemies on the ground.

In-game
In CoD4, 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 dish out pain because of its high firing rate, but it needs to be spindled up and can overheat. Therefore, you'll need to time it so that the Minigun is still revved, yet it doesn't overheat. Remember to keep a constant eye on the gauge on the right side of the screen. If it does overheat and enemies are getting close, you might have to get off it early and run for it, because it takes a while for it to cool down.

Trivia

 * In the game, the minigun is actually placed in the wrong opening. It should be placed in the "window" just behind the cockpit. 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.
 * It's also worth noting that the mini-gun 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.