The M21 Mortar Motor Carriage (MMC) was a self-propelled artillery mount on a half-track chassis used by the United States Army during World War II. It was equipped with an 81 mm M1 mortar and an air-cooled M2 Browning machine gun. It was produced by the White Motor Company in 1944. Only 110 examples were produced.
It mainly served on the Western Front in Normandy, and Southern France, and then later in the Ardennes during the Battle of the Bulge. It was deemed to have been outclassed in 1945 when the 81 mm M1 mortar was found to have insufficient power to provide effective artillery support. (courtesy of Wikipedia)