I've had HE shots actually bounce off the turret of the t29
Yeah, forgot to mention that. As I wrote in a previous post, if your calibre is (if I recall correctly) 3x larger than the armour, it will always calculate as a 90 degree hit.
However, it seems there's also an inverted version of that true, from my experience. HE can and will do no dmg to armour
at all or bounce off against heavy armour. I'm not sure about the conditions, but from what I've tried, an M4 Sherman has a small chance to do no damage at all to IS frontally, though I believe that is really small and depends on angling.
It's especially noticeable when you try using, say, a semi-auto low calibre gun with HE (those things that have like 10 HE pen and such), even against 20mm armour you'll prob just bounce.
To be honest, I can't say for sure, and a quick look into Battle Mechanics page on WoT wiki didn't help, but it's probably based on calibre, BUT! I -think- it also considers gun type/penetration or something too (maybe just armour vs calibre though), since arty seems to be far more reliable than heavies with the same gun (same gun like 105 on Priest and Sherman which has same pen, dmg and calibre, for example).
Prolly also tied with how HE does lower damage to highly armoured enemies. (I can confirm this happens, Obj 212 and S-51 do consistently lower dmg against Maus than against, say, a Tiger II. [about 100-200 less damage, but it isn't the 25% damage randomisation, I'm quite sure.])
Dunno, I'll try and get some practice setions with my Sherman against my friend (he has Obj 704 and KV-3, one has more angling, the other more raw armour..) though if someone has a tier 10 heavy (or some other heavy armour) on EU Server, just post your in-game name and we'll try and test that.
Of course, I'd happily like to hear someone else's views/experiences or real knowledge about this.