[Would you like to see this in-game?]
TL;DR: A KV-1S with a coaxial flamethrower and a 45mm gun in place of the 76mm.
The most common type of enemy a tank can face will always be infantry which, while generally not as dangerous as dedicated anti-tank units, can pose serious issues for any advancing tank, especially if equipped with man-portable anti-tank weapons. As such, it is important for tanks to have ways of dealing with this kind of enemy. Coaxial/hull mounted machine guns as well as high explosive rounds work wonders against exposed or lightly protected infantry but struggle against infantry who have hidden themselves in more fortified structures like concrete buildings or bunkers. Sure, a tank could pelt such fortifications with HE rounds until they collapsed, however, doing so would take a lot of time and a lot of shells, both of which could be put to more effective use elsewhere. For situations like these, flamethrowers are much more effective, however, man-portable flamethrowers are very dangerous to their wearers and result in near certain death when hit by enemy fire. Still, their effectiveness for flushing out fortifications was still very much needed. This need would result in the advent of the flame tank. Early Soviet flame tanks would be based on T-26s and T-27s, however, as the war and German firepower progressed, it was clear that much more protected flame tanks were needed, which would eventually lead to multiple KV-1-based flame tanks. The first of these would be the KV-6 (Note: The validity of this name is up in the air, however, I will continue to use it for simplicity) built in 1941, however, due to an increased demand for KV-1s as well as the ongoing Siege of Leningrad, only 4 out of the 8-10 total KV-6s built would actually be fitted with a flamethrower. On top of that, the KV-6’s flamethrower was hull-mounted, meaning that it would need to point its hull at anything it wanted to burn, a process which generally was not quick. Still, the limited data gathered from the 4 KV-6s appeared to suggest that there was a place for a KV-1-based flame tank on the battlefield, leading to work on an improved design, which would come in 1942 in the form of the KV-8. The KV-8 would be based on a standard KV-1 with a welded turret with the ATO-41 flamethrower being relocated to the coaxial position. This, of course, posed a major issue when it came to space inside the turret. To address this, the standard F-32 76mm gun would be replaced by a 20-K 45mm gun, limiting the vehicle’s anti-tank capability, but not deleting it altogether. The smaller gun would be hidden within a 76mm tube, meant to mimic the F-32, so the vehicle would not draw targeted fire. The KV-8 would be constructed very quickly. So quickly, in fact, that the prototype never even went through factory trials, however, despite that, the KV-8 would be approved for mass production, also very quickly. 300 KV-8s were to be produced by Q2 of 1942, with every tank brigade being equipped with 5 of them, essentially making every other heavy tank a flame tank. Of course, this did not happen and was impossible. Poor quality cast parts as well as a lack of flamethrowers limited the entire production run to only around 42 vehicles. Eventually, the decision was made to upgrade the KV-8 by producing new units on the more modern KV-1S platform and equip the better ATO-42 flamethrower. As such, work began on adapting the KV-1S turret to KV-8 standards, with production of the new KV-8S being approved shortly after, however, due to a lack of KV-1S turrets to modify, production of the KV-8S would be delayed, with a hybrid transitionary variant (KV-1S hull with standard KV-8 turret) being produced instead for a time. Eventually, production of those hybrids was halted to allow for the standardized KV-8S to be mass produced. Unfortunately, switching mass production over to the KV-8S proved very difficult, leading to only 10 being built.
Place In War Thunder:
The USSR is not lacking when it comes to domestic flame tanks. In fact, they may even have the highest number of flame tanks suitable for War Thunder, the KV-8S being one of them. Of course, you could just add a T-34 with a hull-mounted flamethrower and call it a day, however, such a vehicle would play pretty much exactly like its standard T-34 brothers. The KV-8S, on the other hand, cannot play like its standard KV-1 brothers due to its weaker armament. To actually be able to do consistent damage, it would need to get close, which would, in turn, incentivize using the flamethrower more. Thankfully, since its main armament is a 45mm, it would have the fastest reload out of any KV, easily allowing for consistent follow-up damage, and, since the flamethrower is turret mounted, dealing with any open-topped vehicle within 50 meters would be easy. Best placement for this vehicle would be as either a premium or event vehicle.
Armament: 45mm 20-K cannon, 1x 7.62mm MGs, and 1x ATO-42 flamethrower
Dimensions: 6.68m, 3.32m, 2.71m (L,W,H)
Armor: Same as KV-1S in-game
Ammunition: APHEBC, APBC, APCR, and HVAP