This Soviet mechanized artillery is somewhat like the German Stug III because it was directly inspired by it.
Let’s take a quick look at an idea that worked well for both combatants.
The basic idea here is simple enough. This is self propelled artillery that can accompany infantry or other mobile formations. Its mobile so no lengthy set-up period is involved, and its armored in the expectation it will be far closer to the front lines than normal artillery is. Note this is different from a tank in several regards, the most noticeable difference being no turret. The gun is fixed to the hull of the vehicle, that is simpler, cheaper, and allows for a bigger gun on any particular vehicle. And the main weapon is a field gun or howitzer, not specifically an anti-tank gun.
In April 1942 the Red Army requested an armored, mechanized gun after seeing how effective the German Stug III could be. They wanted a common 76 mm field gun and a 122 mm howitzer (a howitzer uses a shorter barrel for high angle fire that can shot behind a wall, into a foxhole, or on the reverse slope of a hill). The 76 mm gun led to the Su-76 on a T-70 tank chassis; while the 122 model was built on a T-34.
Through 1943 638 of them, tagged “Su-122” were built. This was considered adequate for the need, attrition was expected to be fairly slow because of how they were used. The vehicle was built on a T-34 hull, which of course was considered an excellent starting point. This ensured it was reliable and fast enough. The Soviet 122 mm howitzer actually allowed for direct sighting (in addition to its normal indirect fire mode) for an anti-tank function. A HEAT (High Explosive, Anti-Tank) round was developed for the anti-tank role, but it was found the standard High Explosive round had enough mass and concussive explosive force that the special round was not needed.
Keep in mind that a howitzer in direct fire role would not be accurate at a very long range, so this was more of an emergency back stop for when opposing tanks had achieved a break through. The Soviets later built an Su-85 that was a pure tank destroyer on the same chassis. In fact, just like the anti-tank versions of the Stug III, the Su-85 was ultimately built in far greater numbers than the Su-122.
Which leads to saying the Su-122 was only a limited success. It did put a big gun, in an armored shell right with the Infantry. But rate of fire was slow and crew function was clumsy (too many men needed for basic operation). Attempts were made to put a more efficient howitzer on the vehicle, but ultimately this was not ideal either. Post-War a small number of T-54 tanks were built as a Su-122/54, but that is a whole different vehicle.
Those Su-122 produced were expended or worn out during the War and today, only a single example at the Kubinka Tank Museum is known to exist.
This example served at Kursk, Summer of 1943. It is from the Tamiya kit and was a simple build.
Plane Dave 
I don’t know much about tanks so that plethora of serial numbers was mostly news to me (except for the T-34). Years and years ago I read that the Soviets, rather cunningly, never ever used the same number on two different pieces of equipment. I haven’t yet found an exception, although the 76 mm gun on the Su-76 comes close. I suppose once you’d heard the Su on the crackly radio, you always knew what was being referred to.
I’m not sure what that was in reference to. The “Su” designation was always followed by the main gun size. And the T-34 came in a “/76” and “/85” model indicating main gun size.
It could have just meant actual serial numbers, so you couldn’t draw conclusions about manufacturing from them. I know the Japanese pointedly used non-sequential numbers so no one could know how many of a thing they’d built. Or it may mean something about unit numbers and/or radio call signs being random? I don’t know, but I do know different combatants had different protocols for that sort of thing.
Am I right in thinking that the difference between a tank and a self propelled gun is that tanks have turrets ie rotating gun platforms, whereas self propelled guns are fixed? It’s always been something that’s intrigued me!
In a nutshell, yes.
Of course there are more complicated aspects. A tank is usually considered “balanced”. That is its offense and defense are roughly equal. It can defend against the size gun it carries. It is also considered an offensive weapon of break through and maneuver; going through the enemy lines to cause destruction or envelopment to the rear. The modern replacement for heavy cavalry.
By comparison a Tank Destroyer is mostly defensive. It comparatively has an over-sized gun (American tank destroyers had turrets) and is meant to prevent the other side’s armored breakthroughs. Ironically that means its gun is usually bigger than its armor is thick, except is some cases (especially German) for extra thick frontal armor (its not expected to “break through” as much as plug holes.) Like modern pike men.
While mobile guns (or self propelled guns or mechanized artillery or several other synonymous names) are considered support. Usually accompanying infantry (or maybe armor) but not “at point”. They are faster and set up faster so they provide fire support even if the unit just arrived at their new location and aren’t dug in. They virtually never have turrets (again, I know of a few exceptions, but not many). Functionally they are artillery in every sense, although more traditional artillery is cheaper and much slower to set up.
The Su-122 is truly a mobile gun, but the Soviets added an anti-tank function to it (direct or flat aiming capability). So its sort of a hybrid, which isn’t really uncommon with such things.
And of course there’s all sorts of specialized machines and exceptions to the rules, usually meant to fill a specific current need.
Thank you so much for explaining all that Dave. I appreciate there will be many subtle operational differences but you’ve explained the hole thing very clearly. Thank you!
I aim to please!
I definitely built one of these in 1/35th around the same time I did a Stug as part of a pair. As good as the T-34 was as the basic chassis even the early turrets lacked a basket making the crew scramble over ammo boxes and spent shells to operate the tank. There was just something the Soviet’s never quite got about crew-friendly tanks. Even the massive JS series were cramped for the crews to operate in. Another example of one army trying to duplicate the successful weapons of the enemy (even the highly regarded AK-47 is basically a knock off of the Sturm-44 German assault rifle).
Its funny too how the Soviets KNEW their stuff was not very user friendly. They made token efforts to improve things (like padded tape on aircraft interiors to protect from unfinished metal edges), but never in a modern ergonomic sort of sense.
I seem to remember that the early T-34s came with a hammer for the driver to operate the clutch.
Or the take-off procedure for the Pe-2 involved the Navigator stepping up behind the pilot to help pull the yoke back for takeoff.