Later I tried the trick Sethos recommended of going for the shot trap under the barrel, but you have to be freakin lucky to hit that. There was one instance where I put a shell down the barrel of an IS3 and knocked out his gun. He tried to back up but had nowhere to go so I concentrated on trying to hit that shot trap. I put 7 hits downrange with my 90mm right under that barrel and nothin. He repaired his barrel eventually and killed me dead. So I aint a big fan of that strategy!
It is a tricky shot indeed. Just keep in mind that you really can't take that shot from too far out. The shot is meant to bounce off that downward sloping ring on the cannon, into the tank. So even with some distance, the round arching downwards and making the shot impossible. So it has a very limited use and probably more for the situations where you go head-to-head in an urban environment.
That's the problem with the IS-3 and IS-4M, the weak spots and shot traps you are working with are tiny and hard to hit, usually non-existent.