Minigun

The M134 Minigun is a mounted weapon in Left 4 Dead. It is designed for 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 in 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 swiveled in a 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.

In Left 4 Dead 2, the Minigun is replaced by the Heavy Machine Gun, but its sounds are still in the game's files.

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-minigun-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 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.
 * While most people avoid using the minigun these days, there's no replacement for it in finales. If a Tank is hittable with it, hop on and open fire, this will most likely take a large portion out of its health if it doesn't kill it outright. Also, note that shooting the Tank continuously with the minigun will reduce its speed.

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.
 * In Left 4 Dead, Valve used Heavy Weapons Guy's Minigun sounds from Team Fortress 2, another game developed by Valve.
 * 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.
 * Using the Mini-gun is one of the best ways to kill a Witch. If the Survivors are in the generator room and encounter a Witch, it is possible to use the gun to kill it instantly. You do not even startle it, as though you had cr0wned it.
 * In real life, miniguns are specifically designed not to overheat, as the multiple barrels disperse heat equally and allow for indefinite firing. Valve most likely added in the overheating feature so the minigun wouldn't be overpowered.
 * Strangely, the minigun cannot dismember infected, despite firing more bullets of a larger caliber than the assault rifle, which can easily sever limbs and destroy heads.

Glitches

 * Turning on your flashlight while you're aiming the minigun will cause your flashlight to rapidly turn on and off continuously until you dismount.
 * For unknown reasons, one may be randomly knocked off of the minigun in the middle of fire and have some problems getting back on.
 * If there is a oxygen tank, gas can, or propane tank under where you would be standing on the minigun, you will be above the normal height you would be at, and can aim farther down, but less upward.
 * If holding a pipe bomb, first aid kit, or anything that is not a gun the aiming reticule for the minigun will change (must grab minigun while holding one of supplies)