You do realize that the Explosive Rounds completely screws up the impact? Explosive Rounds takes away some initial impact and the adds even more damage than before in explosive damage. Essentially, Explosive Rounds dramatically increases the damage of each shot than without. That Impact Stat is correct that it does more damage than 1000yrd, but the archetype is still the same as shown by the ROF. You also have to put the Exotic Perk into view. This massive damage also accounts for the weapons overall Impact Stat.