Auto Shotgun

The Benelli M4 Super 90 semi-automatic shotgun is a second-tier weapon and is a more powerful version of the pump shotgun that does not require a pump after each shot. It holds a maximum of 10 shells at a time with 128 in reserve, same reserve as the pump shotgun. It is the best short-range weapon in the game, capable of firing extremely fast and taking out massive numbers of Infected per shot.

On both shotguns, the reloading is both a curse and a blessing; it allows topping off the magazine on the fly, and the reloading process can be stopped to handle sudden threats. However, if you are reloading when empty, you will manually cock the shotgun at the end of the reload, regardless or whether it finished or was interrupted. A good trick is to interrupt an empty reload after the first shot by clicking fire then reloading again, as you have gotten the racking animation out of the way and can immediately fire if a threat appears.

The relatively small magazine and low accuracy compared to other second-tier weapons make it slightly less effective against bosses. Although the range on the shotguns in Left 4 Dead is higher than in most FPS games and truer to real life, the range is still quite limited compared to the other firearms. Its high damage output does allow it to seriously wound a Special Infected, but only at a dangerously close range. The exception to this is against Tanks, where the scatter of the shotgun is virtually all offset by the massive target a Tank presents, this generally makes the auto shotgun the best weapon against them due to its massive damage output. Unfortunately, this was seen as too much of an advantage and in a recent update, the damage dealt by the Auto Shotgun versus the Tank was reduced. Its rate of fire can be increased by quickly pulling the trigger as opposed to holding the trigger down, making it an ideal weapon for dealing with large numbers of Common Infected and Special Infected. It is generally a good idea to have at least one member of the team using a shotgun. Like the pump shotgun, it can kill the Witch with one hit if fired into her eyes while she is not attacking, making it useful for getting the Cr0wnd achievement.

Pros

 * The Auto Shotgun is excellent against Hordes of enemies, especially in tight areas like houses or hallways, because they can take out several enemies in a few good shots.
 * The Auto Shotgun has infinite penetration against the Infected. This combined with their spread makes it a powerhouse against a horde.
 * If a Tank or Witch is getting close, your best bet is one of these to dispose of them before they pound you down.
 * The Auto Shotgun is the strongest handheld weapon in the game.
 * The Auto Shotgun can be reloaded with individual shells, so it does not need an entire magazine ejected. In light of this, one can fire and reload a fresh shell immediately without a delayed reload.

Cons

 * If you're surrounded, or in a wide-open area and facing a Horde, this is not a good weapon, as you have to reload often and can't take out many enemies at once if they're scattered.
 * The Auto Shotgun can only hold 10 shells at a time.
 * This weapon is a waste to use while advancing through a Chapter and facing single or small groups of enemies at a time, as most enemies die in a single shot from any weapon.
 * This and the Pump shotgun are the worst long range weapons, as the lead shot scatters. Do not attempt to pick off Infected with this gun, especially Boss Infected.
 * This weapon has the longest reload time, and reloads one shell at a time.
 * With automatic weapons, if surrounded, you can simply spin around in a circle while firing to eliminate all of your nearby foes, but with the Auto Shotgun, you'll take a long time to free yourself from an Infected circle.
 * Friendly fire becomes an issue. On Expert mode, you will almost always incapacitate a teammate if you accidentally fire in front of one.

Tactics

 * Generally refrain from shooting targets at long-range. This is an up-close-and-personal weapon and needs to be used as such for maximum carnage. Wielders of the Auto Shotgun should be given point to maximize its power.
 * Spamming a choke point at close range with the Auto shotgun is very effective against a Horde.
 * Do not hold down the fire key in this situation as doing so will only waste ammo. Since one shot can kill multiple Infected, you only need to tap it.
 * It is also important to note that while the Auto Shotgun has a high rate of fire, firing indiscriminately with this weapon is not always a good idea as doing so will eat away at your ammo supply rather quickly and can raise the possibility of undesirable results, such as friendly fire, or startling a Witch.
 * On any difficulty other than Easy, it is important to remember that the shotgun does have a cone of fire and should your teammates be within that cone, they will take damage as well. Therefore, if you're trying to hit Infected close to your teammate, either use your melee attack against them before shooting or deliberately 'miss' your target to minimize the possibility of friendly fire.
 * On Expert mode in Campaign, if you're using the Auto Shotgun, be extremely careful! One close-range blast is fatal enough to incapacitate one of your teammates instantly. If a blast hits them when they're incapacitated, they're dead. Make sure you're not firing at a Horde attacking an ally on Expert when using the Auto Shotgun. Instead, consider using your melee attack against the zombies to get them off, or crouching and blasting them off with a pistol.
 * There are 12 pellets per shot, equating to 24 damage per pellet, same as the pump shotgun but with 2 more pellets.
 * Any number of Infected can be penetrated with a single shell provided they are lined up conveniently. However, the shotgun does not have the wall penetration of the Assault rifle or Hunting rifle
 * The Auto Shotgun does contain wall piercing fire, and a teammate being mobbed behind a wall or door is easily protected by shooting at them through it. Its stopping power is minimized when doing such, but it is the easiest way to protect a teammate without risk of injuring them.
 * Try not to empty your magazine when fighting the Horde. If you let the magazine reach 0, then you do a completely new animation before you can fire again. In other words, if you let your magazine empty completely, you'll have to wait a second or so before firing, making it annoying and possibly fatal in Horde situations. A smart way to avoid this is to reload every chance you get, and melee if your ammo supply reaches 1 shot left. Since the shotgun loads a shell every half second, using this tactic can keep you shooting without having to sit through the reload sequence.
 * You can still use your melee attack while reloading, which will help if you find yourself in this situation.
 * If you encounter a Tank, feel free to unload into him. It may be the only thing that will keep you alive.
 * If caught reloading by a random horde, the safest thing to do if on harder levels of difficulty would be to melee until finished reloading as cutting the reload sequence could mean when you've emptied the clip you will still have more Infected coming towards you because you only had 4 shells loaded and you were in the middle of a street with open rooms full of Infected so the full reload would help if you were caught in this situation which is attained by meleeing while the reloading continues.

Left 4 Dead 2
The Auto Shotgun returns in Left 4 Dead 2. It apparently has two models; the one seen in Left 4 Dead and a new one, which is a SPAS-12 shotgun. It is unknown if there are any differences between the two.

Trivia

 * The Auto Shotgun is manufactured by the fictional German company Rapidarms, Inc.
 * The Benelli M4 Super 90 semi-automatic shotgun is one of the currently produced shotguns used by the U.S. Military. This may be a reason as to why they are seen everywhere.
 * The Auto Shotgun is based on Counter-Strike's XM1014 automatic shotgun. Its "price" in the gun's weapon file is the same as the XM1014's, and it references the XM1014 in its "animation" listing.
 * The Auto shotgun is always your best bet for taking down Tanks and Witches.
 * The apparent auto-shotgun variant that will possibly be included in Left 4 Dead 2 looks like it was meant to resemble the SPAS-12 shotgun. It was shown at the E3 2009 demo for the game. However, the animation that plays when the character takes out this new Auto shotgun in L4D2 is the same as the L4D animation, making it incorrect as there is no telescoping stock to extend on the SPAS-12. Valve most likely reused the animation either to have the levels ready for the demo on time or without realizing the error.
 * While the Auto Shotgun and the Pump Shotgun are thought to hold the same ammo, the Auto Shotgun technically holds two shots more, as it can hold a maximum of ten rounds at a time and the Pump Shotgun can only hold 8 while both weapons have the same amount in reserve.