First question: 130 mm
Second question: Never, probably

"Object 260 IS-7

The IS-7 heavy tank was developed in 1948.[31][32] Weighing 68 tonnes, thickly armoured and armed with a 130 mm S-70 long-barrelled gun, it was the largest and heaviest member of the IS family.[33] The armour was engineered in a similar fashion to the IS-3, with a pike nose on the upper glacis. When shot at frontally, the extreme angle that the pike nose presents results in a much higher likelihood of a ricochet. Thus, the armour protection could be enhanced without having to use excessive amounts of materials. However, if the pike nose was shot at a sideways angle, it would not have a relative thickness high enough to ricochet the shell. In spite of its weight, it was easy to drive due to numerous hydraulic assists. The loaders noted that the IS-7 was comfortable and that the autoloader was easy to use. It was also able to achieve a top speed of 60 km/h thanks to a 1050-horsepower engine giving it a power to weight ratio of 15.4 hp/tonne, a ratio superior to most contemporary medium tanks. Its armour was not only immune to the Jagdtiger's 12.8 cm Pak 44 but was even proof to its own 130mm. Due to the reasons unknown, most likely because of the considerable issues arising from its mass (bridges, rail transport - no Soviet/Russian tank accepted into service afterwards exceeded 55 t), the tank never reached the production lines.[34][unreliable source?]"

"1946 prototype, only three built. The IS-7 model 1948 variant had a weight of 68 metric tons and it was armed with the 130 mm S-70 naval cannon (7020 mm long barrel). The automatic loader can achieve up to 8 rounds per minute. Other equipment included stabilizers, infrared night scopes, and 8 machine guns. The hull armor was 150 mm placed at 50-52 degree angles. On the turret, the frontal thickness was 240–350 mm at an angle of 45-0 degrees. The IS-7 had a crew of five, with the driver in the hull, the commander and gunner in the front of the turret, with both loaders in the rear of the turret. A Slostin machine gun was to be installed as its AA armament.[33][34]"
https://en.wikipedia.org/wiki/IS_tank_family

http://www.tanks-encyclopedia.com/coldwar/USSR/is-7-object-260

/ Robin