Minigun

The M61 Vulcan Minigun is available for most defending intense, stand-off areas. The miniguns were presumably left behind by the military, but were abandoned when the army left or was killed. It has a small wind-up time, but once it starts firing, it becomes one of the best support weapons of the game. The gun is very powerful but can lose its accuracy at longer ranges. It is bolted to the floor so it cannot be moved, only swivelled in an 170° arc in front of the gun. It has infinite ammo but can overheat; this is shown as the minigun's barrel quickly glows red, as well as the emission of smoke. This smoke also impairs your vision, causing you to have a harder time shooting accurately. However, this is not an imminent signal of the minigun jamming, as it takes 20 seconds of continuous firing before overheating.

Tactics

 * Always have a buddy covering your back, because without being able to rotate the weapon 360°, you are vulnerable from behind while using it.
 * Aim for the masses or the mini-bosses. Don't bother shooting at the one or two infected stragglers if, for example, there is a hunter in clear sight. Your gun can take out enemies the fastest in close quarters. Use it to trim their numbers or take out the big guys. Let your non-mini-gun wielding friends take down the stragglers.
 * When shooting into masses of infected, keep an eye on where your fellow survivors are by looking for their colored outlines. There's nothing worse than getting gunned down by your own ally.
 * When fighting a Tank, aim for the concrete that he throws at either you or the other survivors, as it can be destroyed before it lands.
 * Even when no infected are present, you should constantly tap the fire button. This will keep the barrel spinning, and eliminate the long wind-up time when a horde suddenly comes at you.
 * The minigun also acts as a bait for Tanks in co-op mode, allowing someone to distract the Tank with the minigun, while his teammates pour lead into the Tank.
 * The minigun does not have vehicular properties, meaning you still can be dragged by a Smoker or pounced on.

Trivia

 * The rate of fire displayed by the in-game model, roughly 40 rounds per second, would be considered tame in the realm of modern gatling-pattern weapons. Many can be adjusted to fire up to 100 rounds per second, especially those mounted on aircraft. Even though the rate of fire in-game may seem excessive, it is not all that wearing on the weapon, as each barrel would fire roughly 7 rounds per second, less than some of the hand held weapons in-game.
 * The fact that these guns are in the various areas hint at how desperate the military must have been; any regular machine gun would've been able to do the job with less maintenance, not need a power supply, and be easier to source. Likely, the military set these up to deal with mutations among the infected like the Tank.
 * Valve used sounds from TF2's heavy minigun to use for the minigun in L4D.
 * The minigun does about 156,000 damage per minute, even though no infected has that amount of health.
 * The M61 Vulcan uses 20mm ammo, and is not technically a minigun. Miniguns fire 7.62mm rounds. The reason miniguns are called miniguns is because of their scaled down ammo, 7.62mm being much smaller than 20mm.

Glitches

 * Prior to the January 14, 2009 patch, hitting the minigun repeatedly with your melee attack and jumping onto the minigun would launch the player a certain distance, depending on the number of times you hit it. By using this glitch, it was possible to skip decent sized chunks of many maps, including the forklift crescendo in Death Toll and the door in No Mercy, as well as get to out-of-reach places. The top of the plane in the Runway Finale where the common infected could not reach the players was one place the players could get to by using this glitch. This glitch would work with any of the miniguns found in the game. The number of times you hit it with melee had an effect on the power of the launch. The way you were facing while you were launched had an effect on the direction you were thrown. The direction you used your melee attack from also seemed to have some effect on direction, and hitting the gun from the left seemed to launch you straight forward.
 * Turning on your flashlight while you're aiming the minigun will cause your flashlight to rapidly turn on and off continuously until you dismount.
 * Using the Mini-gun is one of the best ways to kill a Witch, if you are in the generator room and you encounter a Witch, just hop onto the gun and you will kill it instantly! You do not even startle it, it's like you have cr0wned it, No evidence has been shown that this counts towards the Cr0wnd achievement. Using the mini gun to kill the Witch may count toward the achievement. Some people have gained their Cr0wned achievement through this method.
 * For unknown reasons, one may be randomly knocked off of the minigun in the middle of fire and have some problems getting back on.