the one in between the wooden sniper rifle and the regular auto shotgun. idk its actual name i've just heard it called boomgun
The Spas-12? (Which is also referred to as the 'Combat Shotgun')
According to the Left 4 Dead wiki, the Auto Shotgun fires 2 more bullets than the Combat Shotgun, but does 5 less points of damage per bullet, but in comparison to the Combat Shotgun's damage output of 28 per bullet with only 9 bullets per shot, the Auto Shotgun does a whole whopping
1 point of total damage extra per-shot than the Combat Shotgun
So the Auto Shotgun is actually a better tier 2 weapon than the Combat Shotgun, but the Combat Shotgun could be better in that if you aren't shooting the zombies with every single bullet per shot, at least the ones that do hit will do slightly more damage than with the Auto Shotgun