Let’s take a small break from the seething masses for another Ensign’s Q&A! You can find the previous onehere.

Q: What was the historical ROF of the D-25T? Was it with two-piece or one-piece ammunition?

A: The IS-2 was capable of 4-6 RPM. As far as I am aware, tests of one-piece ammunition with the D-25T were only performed on the T-44-122.

Q: How far did nuclear tank projects go? Are there legitimate sources on these projects?

A: As far as Soviet tanks, there weren’t any. The closest thing developed was the TES-3 mobile nuclear generator, designed to deliver power to remote regions.

Q: Why were multi-gun tanks abandoned?

A: Throughout tank-building history, there have been several attempts at multiple guns and multiple turrets. The first such attempt was in infantry support tanks. For example, the Vickers 6-ton (later adopted as the T-26) had a variant with two machine gun turrets. Both of these turrets could operate independently, and cover a sector of the front. When tanks were still few in numbers, and were not likely to meet each other (especially Soviet tanks, whose main enemies at the time were bandit gangs in the Far East), this solution was considered very good. Later, a 37 mm gun was placed in one of the turrets in order to boost the firepower a bit. This tank was not so great at fighting other tanks, since the gun was still low powered, and could not cover 360 degrees due to the other turret being in place. When the time came to upgrade to a 45 mm cannon, the two turrets became one. The same is seen on the Vickers model with the 47 mm gun.
While the effect of 45 mm high explosive shells boosted the tank’s anti-infantry capability, the military wanted more. 3 inch howitzers were common in armies of the day, and many countries decided to equip their tanks with them. However, recoil devices were not quite up to par with the power of these guns, and so they had to be very short. Short guns mean low velocity, low velocity means that these tanks are helpless against other tanks. For tanks like the T-28, this was not much of a problem, since it was intended to fight infantry. In that same vein, it received two machine gun turrets, like the early T-26, since they could now operate independently without interfering with the main gun. However, for the heavy T-35 tank, this was a problem. The tank was meant to break through enemy lines, and defeat tanks and infantry alike. With the lack of a high velocity 76 mm gun, two 45 mm turrets were added.
Less than a decade later, the T-35 was set to be replaced by one of three tanks: the SMK, T-100, or KV. The SMK and T-100 inherited the T-35′s multiple turreted-ness, although now they had a high-velocity 76 mm gun. The KV had both guns in one turret. The SMK lost a turret during development, but, in the end, the verdict was clear: commanding two, or even three, guns was very complicated for the commander. The low caliber guns no longer had any advantage over the high caliber gun. Plus, getting rid of the turret would allow to increase the armour thickness. The KV, with a single 76 mm high velocity gun, won out in that competition.
Throughout the war, multiple guns are seen on one other tank, the Maus. A faster firing 7.5 cm gun is added to defend the tank from infantry, since the main 12.8 cm gun fired exceptionally slowly. Initially on a separate turret, the gun migrated to a coaxial position by the end of the tank’s development life. While reducing the tank’s height, it made it fairly useless. The slow rotation of the turret would mean the gun would be unable to keep up with agile infantry movement.

Now, that’s just for different guns. Having two of the same gun pointed in the same direction was a much more successful tactic. Military history is full of double, triple, or quadruple cannons of various calibers used as AA guns. The Soviets had a prototype SU-122 with two howitzers. Even though it didn’t pan out, its spiritual successor, the Koalitsiya (Coalition) is doing well. There’s also the ST-II tank project, which was quite curious, but died due to the inventor having few friends in the right places and the general declining interest in heavy tanks.