M21 Mortar Motor Carriage

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.

M21 Mortar Motor Carriage
Class Vehicle
Type Self-Propelled artillery
Manufacturer White Motor Company
Origin United States of America
Country Name Origin Year
United States of America 1944
Country Name Operational Year Retirement Year
United States of America 1944 1945 View
ManufacturerName Production From Production To Quantity
White Motor Company 1944 110 View

The M21 had a different layout from the M4 and M4A1 MMC because it was based on a different halftracks (the M4 was based on the M2 Halftrack while the M21 was based on the M3 Half-track). The major difference was the location and direction of the mortar (the M4's mortar was rear-facing and was near the back, while the M21's mortar was front-facing and was closer to the front). Other changes were the differences between the halftrack that they were based on.

The mortar was able to traverse 30° either side and elevate from 40 to 80°. If required, it could be de-mounted from the vehicle and fired from the ground. In addition, there was a pedestal mount at the rear for a .50 cal (12.7 mm) M2 heavy machine gun.

The initial M4 was designed as a mortar carrier and the mortar was fired from the vehicle only in emergencies. 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. This caused problems when the troops who operated the mortar mounted the vehicle, so the US 2nd Armored Division relocated the mortars so that it could fire from the front.

The Ordnance department followed suit and a new 81 mm MMC, the T19 MMC, was created. The T19 went through its developmental trials, which were completed in July 1943, and the vehicle was later standardized as the M21 MMC. Unlike the M4 mortar carriage, which was based on the M2 Half-track, the M21 was based on the M3 Half-track with a longer and roomier rear.

The M21 served on the Western Front seeing action in Normandy and in southern France, before later being used during the Battle of the Bulge, the Battle of Belgium, Operation Market Garden, and the invasion of Germany from the west. The M21 served with the US 3rd, 1st, and 7th Armies during the campaign in France, and the 2nd Armored Division, which developed it. In addition, 57 examples were leased to Free French forces.

Type Self-propelled artillery
Place of origin United States
Service history
In service 1944–45
Wars World War II
Production history
Designer Ordnance department
Designed 1942–43
Manufacturer White Motor Company
Produced 1944
Number built 110
Weight 9 tons
Length 20 ft 9 in (6.32 m)
Width 6 ft 5 in (1.96 m)
Height 7 ft 5 in (2.26 m)
Armor Front:12 mm (0.47 in)
sides: 6 mm (0.24 in)
81 mm M1 mortar
0.5 inch (12.7 mm) M2 machine gun
Engine White 160AX, 6,330 cc (386 in3) 6-cylinder, gasoline (petrol)compression ratio 6.3:1
128 hp (95 kW)
Power/weight 15.8 hp/ton
Suspension tracks: Vertical volute springs; frontleaf spring
Fuel capacity 60 US gal (230 l)
150 mi (240 km)
Speed 45 mph (72 km/h)

End notes