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 L4D is higher then 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 boss, 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 Zombies. 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 and Cons
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.
 * If a Tank or Witch is getting close, your best bet is one of these to dispose of them before they pound you down.

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 bullets scatter. Do not attempt to pick off Infected with this gun, especially Boss Infected.
 * This weapon has the longest reload time, and reloads one bullet 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.

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 melee them away before shooting or deliberately 'miss' your teammate to minimize the possibility of friendly fire.
 * The Auto shotgun is the most effective weapon for dealing with a Tank in close quarters. Players wielding an auto shotgun are capable of bringing down a tank in 12-30 shells, depending on the difficulty level, provided he stays in the right range. Due to a recent update that reduced auto shotgun damage to Tanks, this is no longer the best weapon for dealing with them. The best strategy is usually to run from the tank and pelt it with assault rifle fire, or, if you have one, to light it with a molotov or gas tank.
 * 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 clip when fighting the Horde. If you let the clip reach 0, then you do a completely new animation before you can fire again. In other words, if you let your clip empty completely, you'll have to wait a second or so before firing, making it annoying and possibly fatal in Horde situations.
 * You can still use your melee attack while reloading, which will help if you find yourself in this situation.

Trivia
The Auto Shotgun is manufactured by the fictional German company Rapidarms Inc.