Tactical 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 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. It's 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 can kill the Witch with one hit if shooting her in the eyes while she's not attacking, making it useful for getting the Cr0wnd achievement.

In reality, the M4 Super 90 fires in semi-automatic, not full auto as in game. Very few fully automatic shotguns have been produced.

A very usefull 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.

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.