Just think about it this way (as it is specified in similar games, but HS unfortunately is a little silent about its rules):
There are two triggers, "deal damage" and "receive damage". Piercing Shot, as well as spell damage for example, references the "deal damage" trigger. So Piercing Shot will deal 6 damage to a minion and compare that with the minion's health total to determine the excess damage that is then dealt to the enemy hero. After that "receive damage" is triggered, which gets modified by Mo'arg Artificer and the minion recieves, what, 8 damage, I guess?
Another instance, where the implicit use of those keywords becomes relevant, is with lifesteal. Apparently, lifesteal references "recieve damage" and therefore gives OTK DHs the chance to do what they do.
The third case would be divine shield. I think I haven't witnessed this case yet, but I would imagine HS resolving it like this: A minion with, say, 4 health and divine shield gets hit by Piercing Shot, so it is dealt 4 damage and the enemy hero is dealt 2 damage. Then, divine shield cancels "recieve damage" for the minion, while the enemy hero receives 2 damage. Can anyone confirm that this is what's happening in that situation?
Moonfang would be another interesting case. If my take on the mechanics is correct, casting Piercing Shot on Moonfang, should end with Moonfang recieving 1 damage and the enemy hero recieving 3 damage (without buffs and spell damage, of course).
"deal 6 damage to a minions health. additional effects other than deathrattle effects are not considered. deal the remaining damage to the enemy hero. (ps: you can target your own minions to deal damage to the enemy hero)."