Abstract: Disclosed is an autoloading shotgun firearm which shares interchangeable fully compatible modular sub-assemblies, ergonomics, parts commonality, and exterior appearance with the AR rifle platform. Rapidly reversible tool less conversion from rifle to shotgun and vice versa is accomplished by substitution of the AR rifle upper receiver and ammunition detachable magazine with fully compatible shotgun upper receiver and ammunition detachable magazine. Said shotgun utilizes standard rimmed shotgun ammunition via features in the detachable magazine which securely maintain the position and facilitate the feeding of the cartridges during normal operation. No alteration of the AR rifle lower receiver is required. A short-recoil action is integrated into the shotgun upper receiver. The external appearance and functionality of the AR rifle are retained while eliminating the complexity and inherent fouling issues of typical gas systems used in the AR rifle.