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 one 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.
The specifications for the M21 are similar to the specifications for the M3 Half-track. The vehicle was 20 ft 9 in (6.32 m) long, 6 ft 5 in (1.96 m) wide, and 7 ft 5 in (2.26 m) high with a wheelbase of 135.5 in (3.44 m). The suspension for the front wheels were leaf springs and track's suspension used a vertical volute spring. Powered by a White 160AX, 128 hp (95 kW), 386 in3 (6,330 cc), 6-cylinder petrol engine with a compression ratio of 6.3:1, the M21 could reach up to 45 mph (72 km/h) on a road. The fuel capacity was 60 US gal (230 l) and the vehicle had a range of 150 mi (240 km). It had a power-to-weight ratio of 15.8 hp/ton with the vehicle weighing nine tons.