The IS-3 suffered from a number of shortcomings, both in terms of design and execution. For instance, it had no headroom in its turret and a mere 3 degrees of gun depression. It was also unreliable, something that its predecessor was not. However, there were too many advantages to both the frying pan turret and pike nose hull to simply give up. Zhosef Kotin, therefore, redesigned the tank from the ground up, lengthening the chassis and making the turret a bit taller, increasing the gun depression to 5 degrees. This design, originally called IS-5, was ready for production as Object 730, but it was shelved in favour of a few alternatives, most notably Nikolai Shashmurin’s Object 260.
Object 730 was not revived until after the rejection of the IS-7, and it was re-named IS-8 as a result. After a few minor tweaks to the design, it was finally approved for production as the IS-10 in 1953, but in the wake of Stalin’s death that same year, it was renamed T-10. Initially, the T-10 had the same 122mm D25T rifled tank gun as all its predecessors, with the exceptions of IS-1 and IS-7, but later versions introduced a semi-automatic breach block and a bore evacuator to clear out the fumes faster. A modernised version of the T-10 entered production in 1957, which was equipped with the longer M-62-T2 rifled tank gun, a 750HP engine, and NBC protection. The T-10M was produced as Object 734 at the Chelyabinsk tractor factory, and as Object 272 at the Kirov Plant in Leningrad. Hilariously, tanks produced at different factories had incompatible parts, at least until Object 272 was made the standard in 1962. In 1967, the T-10M was supplied with APDS and HEAT ammunition in a vain attempt to make it as powerful as the T-62 medium tank, but nothing could change the fact that heavy tanks had already been obsolete for a while. The T-10M was the last heavy tank to serve in the Soviet Army, though it was not actually retired until 1993, two years after the Soviet Union collapsed, making it one of the longest-serving tanks of all time, and certainly the longest serving heavy tank of all time (unless you count the IS-3 that rebel forces commandeered during the Donbass war).
Object 730 (IS-5, original engine deck)
Object 730 (T-10, modern engine deck)