The maps in Call of Duty Advanced Warfare are quite large meaning that you need a weapon with good range to be able to compete with others. Of the three shotguns the Bulldog has among the best range and can be shot quickly even though it is a semi-automatic shotgun.
Bulldog best Class Setup
The Best Gun in Advanced Warfare
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The Bulldog is the best Shotgun in Advanced Warfare
Of the three, on balance, the Bulldog is the best shotgun in my opinion. It does have very few bullets per magazine, just 6, and getting extended mags is the most difficult of most weapons, you have to get 25 double kills with the Bulldog which is very difficult to obtain. However, the Tac-12 often needs more than one shot to kill and there is a slight delay between shots. It also doesn’t have more range than the Bulldog.
The S-12 has very poor range and even with Advanced Rifling at ranges you would expect to easily be able to win a gunfight you will often loose. That is still somewhat true with the Bulldog, but if you aim down the sights of the Bulldog you can get a surprising amount of range and the fast fire rate makes it easy to take on multiple enemies at the same time.