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Yes, bad luck. Frontally it should not be able to deal with an IS2, but 50% chance to penetrate the rear armor at close range is fine for a T3 medium armor. Although I have to point out that PzIV is fine, but if you want to argue about IS2 and it's performance, that's another thing. The over performance of the call-in meta is obvious.
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Things got a little mixed up here. Are we debating Ostheer's T3 viability, PzIV or Soviet Call-ins?
Allow me to start with this:
What if you could not finish the game early with the T70 rush (which with a good micro, you can as you said)? If the Axis player is only half decent, chances are you will probably lose the game because your T3 tier is lackluster, and it's gonna take a lot of time to wait for call-ins.
About T34/76, I don't want it to actually counter a panther outright. I just want it to become more powerful with vet, in a way which doesn't make them totally helpless versus a Panther. This is very important when we are talking about "fixing the call-ins". People simply ask for call-in nerfs (by increasing CP and/or price) but that's not the only change we need. If you nerf call-ins, people are forced to use stock units (which opens more room for variety in the game), so you have to give them better options. So both nerfing the call-ins AND tweaking stock units and tiers is the answer.
So bottom line what I'm saying is I actually want to see your vision of T3 (which is them being a bridge between T1/2 and call-ins), but that needs two major changes:
-Fixing Ostheer's teching (Unless we fix this, any buff to Soviets mid and late game will utterly crush Ostheer)
-Buffing stock units of the Soviets
-Nerfing the call-ins.
PS:I also disagree with PzIVs always beating Shermans, specially with Shermans' super duper accuracy on the move.