Auto Shotgun

The Benelli M4 Super 90 semi-automatic shotgun (or colloquially, the "Result Gun") 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 10 shells at a time with 128 in reserve and, like the pump shotgun, can be fired without loading all the shells. This is the best short range weapon in the game, capable of firing extremely fast and taking out massive amounts of infected per shot. However, the comparatively low magazine and accuracy (versus the assault rifle or submachine gun) make it slightly less effective against the Tank or the Witch. Its high damage does allow it to seriously wound a Tank or Witch, but only at "suicidal" range. 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. It helps to know this weapon appeared in Resident Evil 4 with the name "Riot Gun," which is exactly what the weapon is for in Left 4 Dead.

Tactics
When using the auto shotgun,try to keep at least 1 bullet in the gun to prevent a cocking action, doing this will save you precious time.Another very useful tactic is when a boomer vomits or explodes on you or a team mate, go to a corner or a small hallway, you can use the auto shotgun to take down as many infected as possible.