You're also not taking into account the heavy flak's arming time, the heavy flak is a long range weapon it isn't nearly as effective at close range due to the fact it's only dealing a portion of it's possible damage, there are a few ammo types that do in fact where a weapon will deal it's damage but even if we account for those, at long range if the other captain is expecting it they can avoid it. Considering that a gat is a close range weapon and has a fairly hefty magazine, missed shots aren't that important, however with a heavy flak every shot must connect for it to deal any damage against hull armor, otherwise an engineer can just repair it.
If you think I am advocating replacing the Gatling on your metamidion with a Typhon, you are missing the point.
The question was about shooting a mortar before the armor drops. I was making a point about the overall effectiveness of the Explosive damage type against armor, which is to say that it is most certainly non-negligible.