The AA-12 (Auto Assault - 12), originally designed and known as the Atchisson Assault Shotgun, is a fully automatic combat shotgun developed in 1972 by Maxwell Atchisson. However, the original development by Atchisson seems to have produced only a few guns at prototype-level, with the development that ultimately led to the gun entering the market being done later by Military Police Systems, Inc.^[6] The most prominent feature is reduced recoil. The 2005 version was developed 19 years after the patent was sold to Military Police Systems, Inc. The original design later led to development several comparable firearms of such utility, including the USAS-12 combat shotgun. The shotgun utilizes fully automatic blowback action as its primary and only mode of fire. However, the relatively low cyclic rate of fire of around 300 rounds per minute enables the shooter to fire individual rounds through the use of short trigger pulls. It is fed from either an 8-round box magazine, 20-round drum magazine, or a 32-round drum magazine. The charging handle is located at the top of the gun and does not reciprocate during firing.

In 1987, Max Atchisson sold the rights of the AA-12 to Jerry Baber of Military Police Systems, Inc., Piney Flats, Tennessee.^[7] MPS in turn developed the successor simply known as Auto Assault-12, which was redesigned over a period of 19 years with 188 changes and improvements to the original blueprint, modifications included changing the AA-12 from blowback- to gas-operated with a locked breech. Upon firing a round, around 80% of what would normally be felt as recoil is absorbed by a proprietary gas system. A recoil spring grabs another 10%, leaving the final felt recoil only 10% of the normal 12-gauge round.^[8] MPS also teamed up with Action Manufacturing Company and Special Cartridge Company to combine the gun with FRAG-12 High-Explosive ammunition as a multifunction weapon system.

The weapon was lightened to 4.76 kg (10.5 lb) and shortened to 966 mm (38.0 in) but retained the same barrel length. The CQB model has a 13-inch (330 mm) barrel and is half a pound lighter than the regular model. Uncommon in other automatic shotguns, the AA-12 fires from an open bolt, a feature more commonly found in submachine guns, as well as heavy and squad-level machine guns. The weapon uses an Advanced Primer Ignition blowback which strikes the shotgun shell when moving forward before placed in-battery (fully chambered), overcoming the forward inertia of the bolt and offering low recoil. It uses 8-round box or 20-round drum magazines, as opposed to the original 5-round box magazine. Due to the abundant use of stainless steel and the designed clearance for fouling, MPS has stated that the weapon requires little to no cleaning or lubrication.^[9] The designer states that cleaning is required after 10,000 rounds.^[10] A rail system is also available for modern sighting options.^[11] Because of an open bolt design, the AA-12 can reportedly operate after being submerged in water.^[12]

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The AA-12 is capable of firing various types of 12-gauge ammunition such as buckshot and slugs.

By 2004, ten firing models of the AA-12 had been produced^[13] and were demonstrated to the United States Marine Corps, who did not adopt the weapon.^[14]

The Hammer unmanned defense system by More Industries proposed to use dual-mounted AA-12s on the H2X-40 turret.^[15] Neural Robotics also wanted to mount the weapon on their AutoCopter unmanned aerial vehicle.^[16][17]

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