The AA-12 is a non-canon, automatic shotgun featured in some canon maps.
Daniel Smith[]
Apocalypse II: Left Behind[]
The AA-12 in the Apocalypse II: Left Behind storyline is fully automatic, which makes it unique. The AA-12 first appears in That's No Sandstorm, and from then on appears in every mission. This shotgun has a very high rate of fire, medium penetration, and medium-high damage. It can only be obtained from the Mystery Box. When Pack-a-Punched, the AA-12 becomes the "BB-24", which is a play-on of its original name.
Origins[]
The AA-12 in the Origins storyline is also fully automatic. The AA-12 makes its first appearance in Arrival, and appears in every mission from then on. The stats of the AA-12 are exactly the same as the stats from the Apocalypse II: Left Behind storyline. It can only be obtained from the Mystery Box. When Pack-a-Punched, the AA-12 becomes "The Awesome Shotgun". After the upgrade it has a twenty round magazine, ACOG Scope, increased range, and more power.
Remember When...[]
Note that this weapon was cut from the final game.

The AA-12 is a cut weapon from the Remember When... storyline maps and was going to be availiable from the Mystery Box. It can still be used via "give-all" or console commands, however, it does not have a pack-a-punched version, showing that it was cut early in production.
Dark Tears[]

The AA-12 appears in the Dark Tears storyline. It is a fully automatic model, using a completed version of the AA-12 model from Total Warfare: More Than War's single-player campaign. It first appears in Vengeance, and is the primary weapon of some members of the Inner Circle. It's damage is high, as is it's recoil and fire rate. When pack-a-punched, it becomes The Impaler, increasing it's damage, fire rate, ammo size and clip size. It's reload is also reduced significantly, and the gun receives an upgraded sight.
Casted Shadows[]

The AA-12 is scheduled to appear in the Casted Shadows storyline.