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. Its rate of fire can be increased by quickly pulling the trigger as opposed to holding the trigger down. 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.

Tactics

 * In the case wherein you run out of ammo and the player automatically reloads, the character can somehow reload while meleeing enemies. This is very helpful when it comes to the finale part.
 * 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.
 * This means wielders of Auto shotguns should always have point (be in front of the other survivors).
 * 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; since this weapon is so effective, you only need to tap it to blow away multiple Infected at a time.
 * 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 higher difficulties, 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. In other situations, running behind a tank with an auto shotgun while it chases another survivor works extremely well until he begins chasing you. Two survivors using this tactic can deal with a tank quite well on most difficulties, even if the other two aren't there.
 * There are 12 pellets per shot, equating to 24 damage per pellet, same as the pump shotgun but with 2 more pellets.
 * While reloading, you can still melee infected should they get too close for comfort and it helps when you're in a tight spot, same for the normal shotgun.
 * 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 does contain wall piercing fire, and a teammate being mobbed behind a wall or door is easily protected by shooting at them through the wall/door. It's stopping power is minimized when doing such, but it is the easiest way to protect a teammate without risk of injuring them. (EDIT: only works against doors)
 * It is manufactured by the fictional German company Rapidarms Inc.

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.
 * Some cases have been reported of players being able to reload while meleeing with the Auto Shotgun, but it's a theory that is thrown around a lot.

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 is 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.