Minigun

The GAU-2/A (or M134) minigun is a fixed gun used in aircraft. Firing at up to 4,000 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 (likely the one used in Safehouse) just past the church.

The Minigun can really dish out pain because of it's firing rate and it's very high firing rate, but it needs to be revved 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.