D-30 Gun for the IS-6

In 1944-1945, Soviet heavy tank industry was at its peak. A very interesting vehicle was being developed, the IS-6. Unlike its "predecessors" IS-3 and IS-4, (IS series indexing was not at all sequential), it did not just get a D-25T, but a more advanced gun, the D-30:

CAMD RF 81-12038-681

"On the question of experimental works at factory #9 for the 4th quarter

In addition to the experimental works at factory #9 that I have wrote about in quarterly reports, the following were completed in the 4th quarter:

1. 122 mm "D-30" tank gun with ballistics from the "D-25" tank gun.

Factory #9 completed experiments on the prototype tank gun (under its own initiative), which consists of:

1. D-25's monobloc;
2. D-10's breech and gas block;
3. Balancing mechanism with two springs;
4. Lengthened D-25 mount;
5. Fume extractor mechanism.

The experimental D-30 system was tested by firing 30 special strengthened shells from a special stand.

Before shooting, markings were placed on the D-30. The gun fired 30 shells successfully. After shooting, the markings were not deformed. All mechanisms of the gun perform satisfactorily. The full report on the D-30 test was sent to the GAU Artkom indexed #1595/42s on December 5th, 1944 by factory #9. 

In December, after the tests, the gun was sent to factory #100 for installation in the IS-6 tank and continuation of trials."

The D-25 was considered powerful enough for years to come, and, as such, its firepower was not increased. However, the rate of fire was increased in two ways. One is the improved breech, which makes loading easier. That's not that interesting, at least not as interesting as the fume extractor. Existing IS-2 tanks could already fire fast enough that ventilation of the crew compartment could not keep up. The fume extractor expelled the fumes from the burning gunpowder from the front of the barrel, so they could not rush back when the breech was open after a shot, allowing the tankers to fire much faster without succumbing to CO poisoning.