Hit points is ARMA series bane for vehicles. And anything that uses the engine gets that handicap. =(
Incorrect. Iron Front does NOT use ArmA2's stock hit point system. It uses a penetration model based on contemporary Soviet research. You can fire 20+ rounds from a T-34/76 at the front of a Tiger 2 and do absolutely nothing to it, but set in on fire from the rear in a single shot. The model uses location-specific armour ratings, none of which are changed with successive hits. Rounds ricochet, vehicle fuel loads and ammo loads affect penetration results, the Tiger 2's lower quality armour welding is apparently accounted for...it all works pretty well.
As for ArmA2's armour, stick this in your scenarios and enjoy the results:
http://www.armaholic.com/page.php?id=6968