Big Self Propelled Caliber

Unlike SPGs armed with light or medium caliber guns, the British and Americans had a harder time with heavier weapons. This is strange, since SPGs were initially envisioned to mechanize heavy artillery. The British stalled completely in this direction and work did not progress past several prototypes. The Americans fared a little better. The first production vehicles appeared in the fall of 1942 and went into action in the summer of 1944. This was the Gun Motor Carriage M12, the first American SPG with a heavy gun. This article will cover this vehicle's complex fate.

GMC M12: King Kong on Tracks

One of the best weapons of French origin adopted by the US Army in WWI was the 155 mm GPF cannon, designed by Captain Filloux in 1917. The gun had a simple, easy to service, reliable, and effective recoil mechanism, a significant horizontal aiming arc, and a long range. However, the GPF was fairly heavy. It was no accident that it was one of the first candidates for an SPG.

Lend-Lease Shopping Trip

CAMD RF 38-11369-68

"Report on the question of purchasing American self-propelled guns.

If possible, I think that there is reason to purchase 90 mm AA guns on the chassis of a medium tank (used as an anti-tank gun) and 155 mm guns on the chassis of a medium tank. 

Aside from that, it is necessary to purchase quad-.50 cal machine guns on trucks. Preferably not in small numbers, but up to 1000 at a time, with 10-15 ammunition loads. 

As for the fifty 57 mm guns we were offered, I think there is reason to buy them, but only if we receive no less than 500 units at a later date.

Colonel-General Fedorenko."

Obviously the 90 mm gun on a medium tank is the M36 Jackson, and the 155 mm gun on a medium tank is the M12 Gun Motor Carriage. It is interesting to note that the report is dated June 1943 (the date is cut off), and the Jackson only started production in November of that year. The Americans must have been very forthcoming with their latest tank development information. Ultimately, no M36es or M12s were sent to the USSR. 

The quad-.50 caliber machine guns on trucks ended up materializing as the M17: four .50 caliber machine guns on the M5 US halftrack. The USSR received 1000 of them in 1944. 

M17 AA halftracks on the 3rd Belorussian Front, 1944

Here is the final conclusion for what the Red Army needed for its summer 1943 campaign:

"British and Canadian tanks:

1. Increase the number of ordered MkIII Valentines with improved armament to 2000 units.
2. Cease orders of the MkVII Tetrarch.
3. Complete the current contract for 1000 MkII Matilda tanks, and cease all subsequent orders. All incoming tanks must have 76 mm guns.
4. Begin purchase of the MkIV Churchill tank according to the current protocol for heavy tank regiments.
5. Order no less than 500 Universal APCs with a 13.5mm Boys AT rifle.

American tanks:

1. Complete the current order for 1200 M3 Stuart tanks, and cease all subsequent orders.
2. Do not order the M5 light tank, due to no advantages over the M3 light tank.
3. Complete the current order for 1000 M3 Grant medium tanks. After that, explore the question of ordering at least 1000 M4 medium tanks with a diesel engine and improved armour.
4. Order at least 500 light anti-tank SU-57 SPGs"