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 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 improved M4A1 had a reinforced floor so that the mortar could be fired from the vehicle, but the mounting faced to the rear and lacked traverse.

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.

By 1945, it was declared obsolete, due to the insufficient power of the 81 mm mortar as an artillery piece.

[3][10] Standard US tank and armored infantry battalions had a mortar platoon equipped with either M21s or the older M4 MMCs.

The remaining M21 was stored outside until 2003, after which it was restored to running condition and placed on display in the tank hall at the Heeresgeschichtliches Museum.