It appears to be either the Kondensator 2P or the very similar Oka. Kondensator 2P was similar to the US Atomic Cannon project using a Project 271 chassis (based on the IS-3) and armed with a Grabin SM-54 406mm (16-inch) cannon. The very similar Oka was armed with a Shayvrin 420mm (16.5-inch) breech-loading mortar. Both were designed to use nuclear shells. A handful of each were built in the 1950's but were replaced with tactical rockets.
Thanks for the clue. I convinced myself (rightly) that it was not a Kondensator because of the unusual suspension, which I didn't recognize, and stopped looking.
But from Russian Tanks and Armoured Vehicles 1946- present  by Koch you can just make out the distinctive suspension units on the Oka.
This SP gun/mortar is amazing, and its firepower would be awesome!
It’s typical of Soviet engineering to be able to design something like this, and yet remain mobile!
In my time in the Army, I have seen a battery of American M110 203mm SPH firing, which was unforgettable.
But it would make my day to have seen this ‘Oka’ 420mm firing!
Imagine being near it on firing!
You would not want to have your mouth closed – or bye bye ear drums!!
More so I would not want to imagine being in your command bunker on the receiving end of one of its rounds.
As an Assault Pioneer, I have made some very deep and strong underground command posts. But I do not think any of them would have survived a hit from one of these 420mm rounds!
‘That would really upset your commanding officers cup of tea!’
I would look forward to some more info on these powerful Soviet weapons systems if anyone has it, and some more pics and specifications if possible
The St Petersburg museum where the first photo was taken does have some very interesting exhibits, some looking remarkably like WW1 9.2 inch Howitzers.
The Sov 240mm also appeared in at least one May Day parade in Red Sq. Of course it was a nuclear delivery system and not designed to fire HE. It probably never entered service because missiles were much more suitable for nucs.
Those drawings are describing them as "420 mm" ! Battleship calibre ! The chassis
looks a bit too small to me, or would those guns have used a much reduced propelling
charge ? If the shell would have been nuclera or conventional probably doesn't matter
with regards to blowback, I think.
Your absolutely right, Jemiba. The S-103 was some kind of a "monster howitzer". The chassis had problems and at firing it could happen that the barrel bursted (see picture). In the model fotos can the loading mechanism be seen and the atomic shell. And yes, "Kondensator" and "Transformator" were atomic guns. They were the counterpart of the american atomic M65 cannon (thats why the atomic mushroomcloud in the background of the first picture). In hte Kruchshev era this unsuccesful vehicles were soon replaced by tactical missles. See for example wiki:
Another cool feature based on the same chassis and made for nuclear warfare was the "atomicpower fighting station" (my rough translation). It should generate (nuclear)power on the battlefield and be a falloutsafe basis (see the last 3 pics).
Hi MilwaukeeRoad! Thankx for your informations and pictures; great stuff! But I never claimed that Kondensator/Transformator/S-103 and SAE are belonging to the same program. They only used the same chassis base: The IS-3/T-10.