SU - 122 The SU - 122 ( from " Samokhodnaya Ustanovka 122 mm " ) was a Soviet self - propelled howitzer or assault gun used during World War II . The number " 122 " in the designation represents the caliber of the main armament — a 122 mm M - 30S howitzer . The chassis was that of the T - 34 . Development history . Soviet High Command became interested in assault guns following the success of German Sturmgeschutz III SPGs . Assault guns had some advantages over tanks with turrets . The lack of a turret made them cheaper to produce . They could be built with a larger fighting compartment and could be fitted with bigger and more powerful weapons on a given chassis . However , assault guns could aim their cannons in high degree only by turning the entire vehicle , and were thus less suited for close combat than tanks with turrets . In April 1942 , design bureaus were asked to develop several assault guns with various armament : 76 . 2 mm ZiS - 3 divisional field guns and 122 mm M - 30 howitzers for infantry support , and 152 mm ML - 20 howitzers for breaking through enemy strongholds . A prototype assault gun , armed with the 122 mm howitzer and built on the German Sturmgeschütz III chassis was developed designated SG - 122 . Only 10 of these were completed . Production was halted when the vehicle was found to be hard to maintain and judged to be unsuccessful . Simultaneously , a SPG based on the T - 34 medium tank was also developed . Initially the T - 34 ' s chassis was selected for the 76 . 2 mm F - 34 gun . This vehicle , the U - 34 , was created in summer 1942 at the design bureau of UZTM ( Uralmashzavod – Uralsky Machine Building factory ) by N . W . Kurin and G . F . Ksjunin . It was a tank destroyer with the same armament as the T - 34 , but because of the absence of a turret , the vehicle was 70 cm lower , had thicker armour , and was 2 tonnes lighter . It did not enter production . UZTM then worked on combining features of the U - 34 and the SG - 122 . Initial design work was completed between July and August 1942 . The project emphasized minimizing modifications to the platform and the howitzer . It used the same chassis , superstructure , engine and transmission as the U - 34 and was armed with a new 122 mm M - 30S howitzer from F . F . Pietrow ' s design bureau . This vehicle also used the same gun bed cover and mountings as the SG - 122 , to keep costs low and simplify production . It had 45 mm thick frontal armour . The M - 30S howitzer could be elevated or depressed between −3° and + 26° and had 10° of traverse . The five - man crew consisted of a driver , gunner , commander and two loaders . On 25 November 1942 the first U - 35 prototype was ready . Trials ran from 30 November to 19 December 1942 , and uncovered various faults in the design including insufficient elevation , a flawed shell transfer mechanism , poor ventilation for the crew compartment and the fact that the commander had to assist in operating the gun which made him unable to successfully carry out his other duties . The U - 35 entered service with the Red Army as the SU - 35 ( later renamed SU - 122 ) despite these faults . Production SU - 122s were based on an improved prototype built after trials were conducted . They incorporated several modifications including slightly less sloped front armour to ease production , modified layout of the fighting compartment ( the location of crew member stations and ammunition racks were changed ) , fewer vision slots , and a periscope for the commander . The first production vehicles were completed before 1943 . Production history . SU - 122 production began in December 1942 with 27 vehicles built that month . The original plan for production beyond that point was to produce 100 SU - 122s each month . Production continued until the summer of 1944 , by which time a total of about 1 , 150 SU - 122s had been built . Service history . The first SU - 122s produced in December 1942 were sent to training centers and two new combat units , the 1433rd and 1434th self - propelled artillery regiments . Initially , each of these mixed regiments consisted of two batteries with four SU - 122s each and four batteries with four SU - 76 tank destroyers each . Each regiment had an additional SU - 76 tank destroyer as a command vehicle . It was planned to raise 30 self - propelled artillery regiments operating within armoured and mechanized corps . In January 1943 , the 1433rd and 1434th self - propelled artillery regiments were sent to the Volkhov Front near Leningrad as part of the 54th Army . On 14 January they saw combat for the first time in Smierdny region . After that it was decided SU - 122s should follow between 400 m and 600 m behind the attacking tanks ; sometimes this distance was shortened to between 200 m and 300 m . The use of SU - 76 tank destroyers together with SU - 122s proved unsuccessful . Based on combat experience , the organization of self - propelled artillery regiments was changed ; the new regimental organization consisted of two batteries of SU - 76 tank destroyers and three of SU - 122s , for a total of 20 SPGs . In April the organization of self - propelled artillery regiments was again changed . Separate regiments were created for SU - 76 tank destroyers ( light self - propelled artillery regiment ) and SU - 122s ( medium self - propelled artillery regiment ) . The medium self - propelled artillery regiment consisted of four batteries of four SU - 122s each . Each regiment was also equipped with either an additional SU - 122 or a T - 34 for the commander and a BA - 64 armoured car . This organization remained in place until the beginning of 1944 when the SU - 122 started to be replaced by the SU - 152 , ISU - 122 and ISU - 152 heavy SPGs and SU - 85 tank destroyers . The SU - 122 proved effective in its intended role of direct fire on strongholds . The massive concussion of the 122mm HE round was reportedly enough to blow the turret off even a Tiger I if a direct hit was scored at close range , although longer range penetration against heavier German armor remained poor , a trait shared with the larger 152mm howitzers . The new BP - 460A HEAT projectile was introduced in May 1943 ; however its primitive warhead design was only minimally more effective than brute concussive effects of the old HE shell at close range . At least one SU - 122 was captured by the German Army . A small number of SU - 122s survived the war . Currently , only one example remains , on display in the Kubinka Tank Museum . Variants . The SU - 122 had no variants that went into mass production . The T - 34 chassis of the SU - 122 was further adapted as part of the later SU - 85 self - propelled gun . Towards the end of the production run , a prototype SU - 122 was built with the same ball mantlet as the SU - 85 . ( Zaloga 1984 : 160 – 61 ) . SU - 122M . Even as the SU - 122 was being mass - produced , its design was being refined primarily with an eye to reduce production costs . The M - 30S armament proved poorly suited for purpose , in spite of its prior recommendation by the GAU RKKA artillery committee . The howitzer took a lot of space and required both commander and gunner to operate it in order to fire . Because of this , in January 1943 work started on fitting the SU - 122 with a different howitzer . The prototype SU - 122M was built in April 1943 . It featured a bigger fighting compartment as well as an individual driver ' s hatch . The M - 30S howitzer , mounted on the floor of the vehicle , was replaced by the more modern D - 11 howitzer ( a variant of U - 11 howitzer ) . However , the SU - 122M was not put into production due to a decision to proceed with the SU - 85 instead . SU - 122 - III . Another attempt to create an improved SU - 122 replacement took a SU - 85 chassis and paired it with the 122 - mm D - 6 howitzer , which was lighter and smaller than the U - 11 howitzer . This was unsuccessful , due to the unreliability of the howitzer ' s recoil mechanism and its poor anti - tank capabilities . Subsequent wartime design work on 122 mm self - propelled howitzers was cancelled .