As mentioned before, I doubt the caliber of a tankgun is really that big of a difference compared to ALL other parts in a Division.
But the most important fact is, that you won´t get real historical individuality no matter how many choices you open. In HOI I the legendary "88" never existed. It was the very same gun every nation was able to develop. Btw. not even able but forced to do, because there was no way around it, except for stopping research in that field maybe.
In HOI I a Sherman equaled a T-34, which all of us know to be nonsense. But this is not Close-Combat or Combat Mission. Tactics and individual unit strength count nothing. You say attack, your tankdivisions do attack. With slightly worse results maybe when you lack some techs, but you won´t breakthrough because of 20mm more caliber.
The whole thing is very abstract and by giving the units names and histocal data, HOI simply adds flavor, not realism.
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And for all people out there, that generalize statements like a T-34 couldn´t be penetrated by 50mm guns. Please state the exact gun, ammo, range and angle towards the target. Otherwise the info is no good.
A 37mm ATG, as crappy as it was in 1941 could and did destroy T-34s, even frontal. Just had to let it come close enough, and THAT was the problem. No tank was invulnerable. In HOI terms this means higher losses, but still a chance to beat the enemy, as long as the numbers are favoring you.