Development started in the spring of 1944 due to a shortage of M1918 guns for the M12. The vehicle was based on the M4 Sherman and featured a HVSS-type suspension. The design allowed the 155-mm M1 gun and 203-mm M1 howitzer to be interchangeable (an SPG with the latter was designated the M43). The vehicle first saw service during Operation Zebra. A total of 418 M40 and 48 M43 vehicles were manufactured (another 24 M40s were reequipped).
Time for the successive autoreloading of each shell into the magazine.
Characteristics are specified for Siege mode.
Characteristics are specified for Rapid mode.
Reload time of the first and second guns on double-barreled vehicles.
Characteristics are specified for vehicles with crews trained to 100%.