which model of the Is-2? the 1 with the 122mm main gun? IIRC, the Js-2s (Is-2s) with the 122mm main gun only had HE rounds distrubuted during the war.
The D-25T gun fired a combination round (BR-471 APHE) which proved to be pretty effective against German armor late in the war, but largely because the Germans helped! Due to a shortage of manganese starting in the summer of '44, the Germans were forced to switch to nickel alloyed high-carbon steel, which if you're familiar with metallurgy means
HARD but
BRITTLE. In many cases the JS-2's 122mm didn't penetrate, but shatter the panzer's armor, especially at the welds.
Here's some interesting reading:
http://www.battlefield.ru/content/view/32/47/lang,en/