M4 Mortar Carrier: Standardized in October 1940. Classified as Limited Standard in December 1942. Based on M3 chassis. Carried gun and ammo. Mortar had to be removed for firing. Was sometimes fired from inside of vehicle in emergencies. It faced the rear of the vehicle.1
M4A1 Mortar Carrier: Standardized in December 1942, and classified as Limited Standard in July 1943. Improved with reinforced floor and mounts for firing mortar from within half track. Had extra storage boxes at rear. Mortar fired to rear.1 Velocity of 700 ft/sec, range of 3,288 yards. Had a .30 cal Browning MG installed on the gun rail which went around the top. Also included a bazooka, M1 rifle, and a .45 cal submachine gun.
M21 Mortar Carrier: Prototypes were manufactured by White Motor Co. Had traversing mount and mortar fired forward.1 Traverse of 30° each way and elevation 40° to 80°. Pedestal mount in rear for .50 cal MG. Mortar could be removed for firing. A 2-way radio was also installed.