Abstract: A gas pressure adjusting device in the gas-operated mechanism of a gas-operated autoloading shotgun is in the form of a valve assembly comprising essentially a valve seat encompassing a stepped valve opening consisting of adjacent opening parts of different diameters, a valve body having first and second pressure-receiving surfaces and first and second sliding contact surfaces respectively fitted slideably in the valve opening parts, and a spring urging the valve body toward and into closed state against the valve seat. This valve assembly operates with a double action wherein the gas pressure tapped from the barrel bore acts initially on only the first pressure-receiving surface of the valve body and, upon exceeding a predetermined pressure, moves the valve body past a point where it acts also on the second pressure-receiving surface thereby to be released into the outside air.