The Auto Assault-12 (AA-12) shotgun (originally designed and known as the Atchisson Assault Shotgun) is a firearm that was first developed in 1972 by Maxwell Atchisson, existing in its present form as the 2005 version, developed over 18 years since the patent was sold to Military Police Systems, Inc. The original design was

the the basis of several later weapons, including the USAS-12 combat shotgun. The weapon can fire in semi-automatic or fully automatic mode at 300 rounds per minute (5 every second) and has a magazine of 32 rounds – either a standard box, or a drum magazine.

Specifications
Weight: 5.2 kg less magazine. 7.3 kg with loaded 20-round drum
Length: 991 mm

See it in action:

[via wikipedia & youtube]