The range displayed in your pipboy is the maximum range of your weapon and shooting from beyond maximum range will reduce your damage by 50%.
All weapons also have a minimum range and shooting from inside that range will do full damage.
Between minimum and maximum range the damage will scale from 100% to 50% linear to the distance.
You can measure your range in shack foundations:
- ( Range / 12.0 ) is pretty much the distance in number of shack foundations attached to each other in a straight line.
- tested by shooting at mama murphy sitting at the end of a long "road" of shack foundations: http://i.imgur.com/AsBiFxI.png
- how to mod damage reduction: http://forums.nexusmods.com/index.php?/topic/3452490-fo4edit/?p=35311245