The Auto Assault-12 (AA-12), originally designed and known as the Atchisson Assault Shotgun, is a shotgun developed in 1972 by Maxwell Atchisson. The most prominent feature is reduced recoil. The current 2005 version has been developed over 18 years since the patent was sold to Military Police Systems, Inc. The original design was the basis of several later weapons, including the USAS-12 combat shotgun. The weapon is fully automatic only but fires at a rate of 300 rounds per minute, making it possible to fire one round at a time with brief trigger pulls. It is fed from either an 8-shell box magazine, or a 20- or 32-shell drum magazine.
In 1987, Max Atchisson sold the rights of the AA-12 to Jerry Baber of Military Police Systems, Inc., Piney Flats, Tennessee. MPS in turn developed the successor simply known as Auto Assault-12, which was redesigned over a period of 18 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.
"When the bolt flies back after firing to cycle another 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 recoil a remarkable 10% of the normal recoil for a 12-gauge round—so you can point the AA-12 at a target and unload the full magazine without significant loss of accuracy". MPS also teamed up with Action Manufacturing Company, and Special Cartridge Company to combine the gun with FRAG-12 High-Explosive ammunition into 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 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. It uses 8-round box, 20-round drum, or 32-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 claimed that the weapon requires little to no cleaning or lubrication. The designer states that cleaning is required after 10,000 rounds.
The AA-12 can use different types of 3" 12-gauge ammunition such as buckshot, slugs, or less-than-lethal rubber stun batons. It can also use high-explosive rounds, essentially becoming a man-portable automatic grenade launcher or autocannon.