AA-12

The AA-12 (Atchisson Auto Assault-12) is an automatic shotgun considered to be the first shotgun directly designed for military use. It is available in Call of Duty: Modern Warfare 2.

Call of Duty: Modern Warfare 2
The AA-12 is known to be capable of accepting the Red Dot Sight, Suppressor, Grip, Heartbeat Sensor (only available in single player and special ops), and Jacketed Ammunition (even though shotguns fire pellets or slugs). It has an 8-round magazine capacity (16 with the extended mags), balancing the weapon's power and speed, but may have a capacity of 70 extra shells, and fires full-auto. The AA-12 empties its magazine far more quickly than it reloads as the magazine needs to fit onto the magazine support, and then slide up into the magazine well, though this is slightly offset when using the Sleight of Hand Perk. Since it uses magazines rather than individual shells, it is the fastest loading shotgun in the game. When using the AA-12, try firing it in bursts, and not to hold down the trigger unless completely surrounded in close quarters. Firing full automatic introduces significant vertical recoil, even with the grip. In multiplayer, the AA-12 has only one extra magazine, so use it sparingly (unless you have Scavenger to keep your ammo count up.) It also has moderate recoil and poor range compared to other shotguns, so players will have to get fairly close to use it effectively. Using Extended Mags increases the shotgun's magazine to 16 rounds, allowing the user to fire it for longer. This makes it a very good weapon for clearing rooms full of enemies. Many people have suggested it has the worst range of all the shotguns, having to be near point blank range to cause any harm, and its fire rate is so high you have to keep some control when firing the weapon.

It is recommended that a user uses either Scavenger with the AA-12 - it's ammo runs out after two magazines, or one if the player is using extended mags. If attached with the Extended mags, it is recommended that one uses Bling Pro and adds the grip as full-auto with the extended mags will make aiming almost impossible.

Call of Duty: Modern Warfare: Mobilized
In game, the AA-12 is used in the U.S. Marine missions. It comes with a 32-round drum magazine by default, more damage and slightly slower automatic fire than the console version. It is the second last weapon to be unlocked for the USMC/SAS multiplayer faction.

Tactics
The AA-12 is commonly used as a primary weapon for its incredible power and fire rate. Most people who use this weapon have the perks Scavenger Pro(For infinite bullets), Stopping Power Pro(to insure faster kills), Steady Aim Pro(for better hipfire acuracy). Basically you just run around one-shooting everybody down because of it impressive power. Despite the low range, the AA-12's impressive power and fully automatic rate of fire is enough to kill a person.

Trivia

 * The AA-12 fires faster, has much more range, and lower recoil in singleplayer than in multiplayer.
 * The AA-12 has a different reload when the Heartbeat Sensor is equipped; the empty magazine will actually be thrown away when reloading, instead of disappearing off-screen as is the case with other attachments. The Heartbeat Sensor is also very visible even when aiming down the sights, this charateristic is shared with the TAR-21.
 * The 16 round magazine in multiplayer is unrealistic, as in real life the AA-12s only magazine sizes are 8,20,and 32 round magazine/drums.
 * In reality, an AA-12 carries a 32-round magazine and has a range of about 200 yards. Obviously, however, balancing would be drastically off if this was done in game.