Atk and impact are kind of one and the same but they have variables. There was a nice post somewhere on testing the two and basically some guns will do better having impact and other atk. I believe that impact does have an effect on how much of ur damage goes through armor and atk is like a base stat. Range does affect damage by drop off but it's at pretty far distances (doubt someone wants to snipe a hand cannon)
Some cases atk is valued more in others impact is
That's the gist of it
English
-
Attack and impact are values you can't compare. Attack is the priority because you want to make sure your weapon is EFFECTIVE against certain types of enemies. Low attack on a low level enemy has the same efficiency as a weapon with high attack on a high level enemy. Impact is the amount of damage dealt, in which is altered by the weapons range IN COMPARISON to the enemies level