Lemme explain. 1. The attack stat, the one you're talking about, simply indicates the weapon's ability to deal full damage to an enemy. 2. The impact stat, measured as a bar, indicates the damage per bullet. 3. If you shoot a level 5 enemy with a 50 attack weapon, it will deal full damage. However if you shoot a level 10 enemy with a 50 attack weapon, it will(let's say the impact of your weapon does 100 damage) do less damage, because the weapon's attack is not a high enough level to deal full damage to the target.

Please excuse the double post.