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 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 selective fire, operating as a semi-automatic, or in fully automatic mode at 300 rounds per minute. It is fed from either a 8-shell box magazine, or a 20 or 32-shell drum magazine.

The weapon was lightened to 4.76 kg and shortened to 966 mm 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 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. It is designed to fire three different types of 3" 12 gauge shells: Buckshot, slug, or Frag-12 rounds. Due to the abundant use of stainless steel and the designed clearance for fouling, MPS has claimed that the weapon requires zero cleaning or lubrication.[2] The designer states that cleaning is required after 10,000 rounds.

The AA-12 uses many different types of cartridges such as 00 buck shot, #4 bird shot, 12-bore lead slugs, or less-than-lethal rubber stun batons. It can also fire flares or special FRAG-12 18.5mm fin-stablized HE, HEAP, and sensor fused HEAB "air-burst" fragmentation shells that can detonate in mid-air.