A couple of points:
Default values in the calculator are not representative of normal use for that weapon.
10 seconds is too short to allow for the actual damage difference between the weapons to be seen in the calculator.
Div 9 hasn’t been a recommended weapon for froobs for a number of years.
If you want a good assault rifle, (after the subway ofab AR) check out the Neutrino Flash from Steps of Madness:
Or these: Easy to get, decent weapons