One of the problems encountered was the gas system that operated the rifle's semi-automatic action. By May 1940, Williams had fitted the rifle with a short-stroke gas piston outside the bore of the barrel that used the gas forward of the breech to cause the piston to strike the operating slide and cycle the action. Winchester designated this rifle the Model G30M. The rifle developed with this version of the short-stroke gas piston was used during U.S. Ordnance Department trials at Quantico, Virginia, and Aberdeen, Maryland, from March through April 1940, followed by trials with the Marines in San Diego, California, during the fall and winter of 1940.