AA-12

The AA-12 (Atchisson Auto Assault-12) is a select fire shotgun considered to be the first shotgun directly designed for military and law enforcement use. It fires at 300 RPM, and accepts an 8 round detachable box magazine, a 20 or 32 round drum. Its biggest rival is the versatile, and extremely popular Russian Saiga 12KS-K select fire shotgun.

Call of Duty: Modern Warfare 2
The AA-12 is a unique shotgun because it fully automatic. It has an 8-round magazine capacity, which is doubled when Extended Magazines is attached, which balances 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 (650 RPM singleplayer) 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, as its ammo runs out after two magazines, or one if the player is using Extended Mags.

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 instead of a primary weapon - a lot of playeres have the Riot Shield for back protection - for its incredible power and fire rate. Many people who use this weapon utilize the perks Scavenger Pro, Stopping Power Pro and Steady Aim Pro. (Scavenger Pro would be best paired with the Extended Mags, because of the AA-12's low ammo.)

The AA-12 is a good weapon on multiplayer when entering rooms in order to clear out snipers or campers because of its automatic abilities. This weapon is at its best with the extended mags attachment as it needs constant reloading. Using a silencer is almost completely useless as the range is shortened to about five yards.

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 sight, a characteristic that is shared with multiple weapons including the TAR-21 and the L86 LSW.
 * The AA-12 in singleplayer fires at about 2 times the fire rate as its real life counterpart. In game, it fires at 550-650 RPM, and reality, at 300 RPM. In multiplayer the fire rate is far more realistic.
 * The AA-12 in real life has an optional magazine capacity of 20 shells (though it can accept a drum magazine that increases it to 32 rounds) instead of the game's 8, and the range of the weapon is particularly unrealistic as well as the creator of the shotgun said anything within 200 yards of the gun cannot survive. In-game its range is significantly reduced, down to about 20 yards. These 2 aspects of the weapon in-game are probably for balancing purposes, though a 20 round magazine could be used as the extended magazine attachment.
 * The AA-12 in real life has mini-grenade rounds that when detonated spread out towards the enemy at approximately 10 feet, according to the creator of the gun this translates into essentially being able to send 20 grenades down range towards the enemy in rapid succession and due to the spread of the grenade rounds also extends the gun's firing range by an additional 175 meters as noted by the host of Future Weapons.
 * The AA-12 in real life fires from an open bolt, meaning the bolt should be locked back before firing, then slide forward to fire the shell.
 * The AA-12's extended magazine attachment in-game doubles its ammo capacity to 16 shells. Strangely, there has been no record of a 16 shell magazine ever made for the AA-12.