SU - 85 The SU - 85 was a Soviet self - propelled gun used during World War II , based on the chassis of the T - 34 medium tank . Earlier Soviet self - propelled guns were meant to serve as either assault gun s , such as the SU - 122 , or as mobile anti - tank weapons ; the SU - 85 fell into the latter category . The designation SU - 85 is derived as follows : ' SU ' stands for the - self - propelled carriage , while " 85 " signifies the bore of the vehicle ' s armament , the 85 mm D - 5T gun . Development history . Early in World War II , Soviet tanks such as the T - 34 and KV - 1 had adequate firepower to defeat any of the German tanks then available . By the fall of 1942 , Soviet forces began to encounter the new German Tiger tank , with armor too thick to be penetrated by the 76 . 2 mm guns used in the T - 34 and KV tanks at a safe range . The Soviets also had received reports of a new , more heavily armoured German tank then under development , the Panther tank . Although the Panther was not seen in combat until July 1943 , the new generation of German vehicles meant the Red Army needed a new , more powerful main gun for their armoured formations . In May 1943 , work was begun on a new anti - tank gun . Developers turned to adapting an 85 mm heavy anti - aircraft gun for this purpose . The gun was far too large to fit into the turret of the T - 34 or KV - 1 , but it was thought it could be mounted in the chassis of the SU - 122 . Meanwhile , the SU - 122 was rearmed with an existing 85 mm gun , the S - 18 , which was modified and improved in the process . Initially the production factory at Uralmash attempted to reject the design since the larger gun breech meant that the entire hull would need to be modified , but they were required to do so nevertheless , and the modified design was put into production . Several prototypes of the SU - 85 were created , and rejected for design flaws . After several changes , including another armament change to the D - 5T , one was put into service as the SU - 85 . At the same time , the 85 mm D - 5S , which had proven both effective and reliable , was modified to include a telescopic sight and a new ball mantlet design and retitled the SU - 85 - II . Description . The SU - 85 was a modification of the earlier SU - 122 self - propelled howitzer , essentially replacing the 122 mm M - 30S howitzer of the SU - 122 with a D - 5T high - velocity 85 mm antitank gun . The vehicle was small with good mobility . The trade - off was its relatively light armor . Production history . SU - 85 production started in mid - 1943 , with the first vehicles reaching their units by August . When the up - gunned T - 34 - 85 medium tank entered mass production in the spring of 1944 , there was no point in continuing production of a tank destroyer without superior firepower , so SU - 85 production was stopped in late 1944 after 2 , 050 vehicles had been produced . It was replaced on the production lines by the SU - 100 tank destroyer , armed with the more powerful 100 mm D - 10S gun . There were two versions : the basic SU - 85 had a fixed commander ' s cupola with a rotating periscope and three vision block s ; the improved SU - 85M had the same casemate as the later SU - 100 , with a commander ' s cupola as used on the T - 34 - 85 . Service history . The SU - 85 entered combat in August 1943 . It saw active service in Soviet , Polish and Czechoslovak forces on the Eastern Front until the end of the war . It was obsolescent by 1945 , and was withdrawn from active service not long after the war , to be exported to many Soviet client states in Europe and elsewhere . The SU - 85 remained in service longer in North Korea and Vietnam . The similar SU - 100 remained in service much longer , and some SU - 85 and SU - 100 were converted and used as command and recovery vehicles .