I did testing on this a week or so ago with a zerker in my guild. the Attack Rating number on your persona screen seems to be tied to the damage rating on your weapons and your strength, and your skill with the weapon for the 3rd factor. Swap out a weapon with the same attack rating and better str and you'll see a slight increase. Swap out something with the same str but a lower attack rating (or higher) and you will see a significant change. Watch your attack rating drop when you click on focus fire. Mine drops from like 1150ish to 580 when focus fire is on and I'm using a bow.