Gas operated auto loading firearms, and particularly shotguns, typically comprise a gas cylinder parallel to the barrel of the gun. A bleed orifice extends from the barrel into the initial volume of the gas cylinder, to permit the pressurized gas resulting from the shot being discharged to pass from the barrel into the gas cylinder. The pressurized gas acts upon a mechanism which operated to replace spent shells with live shells from a magazine tube and cock the hammer of the shotgun for the next shot. For most gauges of shotgun, two or more ammunition loads are generally available. A light ammunition load is typically used for target practice, with increasingly heavy charges found in field loads and magnum loads.
In the past, different barrel and inertia sleeve assemblies have been used to accommodate the different ammunition loads, and effort has been directed toward the development of a firearm that would satisfactorily accommodate all of the readily available ammunition loads without modification of the mechanism or shooter adjustments.