then why on earth does it deflect shots from paks and take so little damage from P4s and penetrate so often?
As an aside, If you went up against a doctrine buffed M18 (i did yesterday and they raped my panther) then what you described happens. It's the combo of tungsten tipping and AP rounds T4 that cause it to penetrate so often. Also I think they get a health buff in one of the doctrine selections giving the impression that they "take so little damage." They just have more life.
And this from Mysthalin:
the pak has a base penetration of 0.84 vs M10 armour(which the M18 uses), meaning
hat it will penetrate all the time at close range, but at medium range it falls down to 0.755, and at long range to a further 0.71
meaning that it will bounce 1 in 4 shots